I am sorry if this has been done before, but I haven't found a posting about this in general technical, and thought you all might like to know a simple cheap way to do this!

Here is one I prepared earlier:

(update: sorry I will have to find a web location for the pictures, they don't seem to paste here I will do the post anyway it may still be of use without the pictures.  )

BMW 523i 1998 Self Dimming Rear-view Mirror

Disclaimer first: This is an example of how this can be done, not how it should be done. I take no responsibility if you blast your electrics!

That said, here we go.

Tools:

• 1 Small flat screwdriver
• 1 roll of masking tape or electrical tape
• Nimble fingers
• 1 sharp hobby knife
• Optionally a multi meter
• Optionally a bright torch

Required:

• 1 Self dimming mirror to suit E39 / possibly others (eBay)
• PC CDROM to soundcard wire or 3 wire with the correct size BMW connectors on them, possibly supplied by BMW (I used CDROM to Soundcard wires successfully, but they are a little small and fiddly to align).
• Two snap easy fit wire connectors (the blue ones you clamp together which cut the wire insulation and make a join

This is really simple!

Pull out fuse number 4 to remove power to your interior lights (check that this number matches with your car!)

Carefully ease out your interior light using the small flat screwdriver.

Unplug the light (on the left side of the connector is a tab you need to depress to remove the connector)

Place the light in a safe place.. you don’t want to be sitting on it ;-)

Using your fingernails or the screw driver carefully lift out the plastic plate covering the wiring plug for the existing mirror, it has typical small plastic hook tabs at the top end and latches at the bottom end, so start at the top

<picture of removal plastic insert on mirror>

Unplug the connector; you need to depress the tab to release it.

Carefully ease the rear view mirror off by pulling outwards and downwards towards you, don’t be tempted to twist the mirror it has tabs on the back that can be damaged.

Take the soundcard wire and carefully release the metal sockets from the plastic body of the plugs at either end using a small screwdriver or a pointed knife

Note the numbering of the, the male end of the original mirror plug which is still hanging from the roof, then with a small screwdriver release the sliding cover on the plug, and ease out the internal plug from its shell, again noting which way round it needs to be.

Using the sound card wires, cut of one of the metal sockets leaving three, I chose Red, White and Black cutting off the yellow, Red for power (+12volts) Black for ground (0volts) and White (unfinished in this guide) for reverse power to un-dim the mirror when you reverse.

Carefully insert the Red wire in to the hole for socket / pin 3 in the plug hanging from your roof (you did make a note of the numbering didn’t you?! If not you may need to place the outer back on to check the numbering.

Carefully insert the White wire in to the socket / pin 1 again in the plug hanging from the roof.

Finally insert the black wire in to the socket /pin 10.

This is the fiddly bit as the wires (unless you got the official BMW version) want to slip out again all the time, however once the plastic cover shell is slid back on again it seems to hold the wires correctly, and worked fine for me!

Now tape the other end of the soundcard wires to a screwdriver to help feed them in to the rubber casing alongside the existing two wires, feeding them through to above the interior light, the rubber casing can be eased out to help, but is tricky to insert back in to its old position! Remove the screwdriver!

Align the new dimming mirror with the windscreen fitting, at about 45 degrees to the vertical with the mirror aligned to allow the correct angle and twist to vertical, this might take a couple of attempts to get right, but the mirror will “snap” in to place when correct. Plug in your newly wired connector, checking that the soundcard wires do not slip out or break loose, this worked first time for me though it didn’t seem like it would!

Once you have the wires through you need to splice them to the interior light wires, Brown one for Earth (0volts) to connect to the black soundcard wire, and Red and Yellow to connect to the red soundcard wire

<Picture of interior light plug>

Using the sharp knife carefully nick a small hole in the insulation wire on the Red and Yellow wire for the interior light, then place the red metal socket from the soundcard wire in to the blind hole in the blue snap connector, and aligning the nick in the insulation with the metal plate in the snap connector, insert the red and yellow wire in to the through hole, clamp and close the connector, repeat for the brown light wire and black soundcard wire.

<Picture of completed wiring connections> 

Now to test:

Plug the fuse back in again

Put your finger over the hole in the back of your mirror with the front facing sensor and wait (optionally shine a bright torch at the rear view sensor in the face of the mirror).

The mirror should go blue and dim

<Picture of dimmed mirror> 

If it doesn’t dim, recheck all your electrical joins, a multi meter is useful at each step, to determine you have connected and have power etc. A thin strong metal wire is useful to insert in to the connection holes, again to confirm connection, the mirror connection, set on resistance momentarily will read a resistance before going back to zero (between the new red and black wires)

The meter probe carefully stuck in the cigarette lighter (the top outer metal casing) is good for an earth, but be careful as it is quite easy to short the lighter out and blow a fuse.

 <picture of earth connection using cigarette lighter>

Once the mirror dims correctly you can tape the sound card wires on to the light wires for support, I use masking tape, as I hate the sticky gak caused by insulation tape.

<picture of taped wires ready to put back in to place>

Then push the wires back in to the light hole, and off slightly to the right to avoid heat from the light. Re fit the light.

Fit the mirror plastic casing, and you are done!

What I don’t know yet is the effect on the battery, but as the mirror uses a very small amount of charge, I don’t think it will cause any harm.

When I find the reverse sensing output, I will complete this post. All this will do is clear the mirror when you go to reverse in the dark with a light shining in the mirror!

There is a really good posting on the E60 forum with attached PDF files (you will need to join) with details of the electrical connections and location of the reverse wire… only problem is the picture is for a E60 not an E39… the E39 doesn’t seem to have a little “Rear Driving signal” A3 / X12 box L

Essentially the mirror dims using +12 volts and Earth between pins 3 and 10 respectivly

Enjoy your night driving!!!!

Here are the pictures:

<picture of removal plastic insert on mirror>

Thanks to E60 forum Rudy, I borrowed a screen shot from the PDF on how to remove interior mirrors

http://picasaweb.google.co.uk/jerry523i/BMWStuff/photo#50904 83075314669410

<Picture of interior light plug>

http://picasaweb.google.co.uk/jerry523i/BMWStuff/photo#50904 80451089651522

<Picture of completed wiring connections> 

http://picasaweb.google.co.uk/jerry523i/BMWStuff/photo#50904 80421024880434

<Picture of dimmed mirror>

http://picasaweb.google.co.uk/jerry523i/BMWStuff/photo#50904 80382370174722

<picture of earth connection using cigarette lighter>

carefull how you do this, but on a rainy day a each location close at hand is useful, especially if the probe on the multi meter will jam there and stay put!

http://picasaweb.google.co.uk/jerry523i/BMWStuff/photo#50904 80408139978530

<picture of taped wires ready to put back in to place>

http://picasaweb.google.co.uk/jerry523i/BMWStuff/photo#50904 80395255076626

Have fun