These things seem to have a mind of their own when everything isn't perfect.

Note: The input connector when looking at the mirror upside down with the mirror side facing you is [ POS - GRD - Reverse( Over ride). ] For reference, the pos lead goes to the switch. I put a piece of shrink tubing over the GND post and connected the ground wire to a screw on the metal plate instead of at the connector. I attached a small alligator to the positive post making sure it wouldn’t short to the ground post.
Test the unit with the covers off using a 12-volt power supply and small alligator clips. Be careful that it’s situated in an upright position (you might have to lay it down as well, but keep the area clear of any metal objects!). You need to be able to see and feel what’s going on.

1.) First of all I removed the pulleys and cleaned the teeth and pulley surfaces with alcohol and a tooth brush (I really soaked them and got into the teeth so they were like new), and added lubricant only after they were installed. The idea is to have the belt contact area really clean and a small amount of grease on the gears and very light on the cams. I used fresh belts and washed my hands before handling them. Grease attracts dirt and gets into the belts and contacts. Doing this frees up the drive train and will help the aging motor. This is a micro system so everything makes a difference. When the pulleys were out I cleaned the contacts below with a Q-tip soaked in alcohol. Clean under the contacts good and the area around them. Check that they aren’t bent and will be activated by the cam. (This is where I think your problem is.) I also blew it out with compressed air. Check the that the belts are not slipping (probably not the case but I had to try a couple of different sized belts to get it to work right.) You can turn the pulleys by hand trying to get them to slip. Hold one pulley and turn the other- does the belt slip? Also check that the drive train isn’t binding by hand. I know you already checked this but do it anyways. Check that the cams are activating the contacts below. Also clean the light sensor openings and the rear view sensor window.
2.) Connect just the Pos and Grd as above with the mirror off. Now turn it on. If the motor starts running continuously, then turn it off or disconnect it because I think the motor will burn out. I smell a lot of “stuff “ when mine runs too long. See below on what to do with this – in other words for you – see below.
3.) The mirror may run to day light position and stop. Watch the LED in the middle, it lights up when the mirror is activated (dimmed) and shuts off when it’s not. The square LED is just a light for the dial. I want to find a blue one for this.
4.) If the light stays on (dimmed) and the motor does not run, move the output pulley by hand, back and forth just a nudge. It should move to correct itself until it activates the contact. Now try moving the dial pot (where the dial shaft goes). Be careful with what you use obviously. I found a 1” allen driver for this that didn’t fall through and sat straight up. Plastic would have been better. If the mirror is dimmed (led on), it should return to daylight position with the room light on and the dial pot moved towards far I believe. Just move it one way and then the other to try to get it to move. You may have to wait approximately 10 sec for it to do so. So move it and wait, then the other way and wait. Now the mirror should be in daylight position.
5.) Now turn the lights out! Wait a bit (Note this: The mirror has an approximate 10 second delay that can really mess things up at times. I meant approximate because it varies between 5-15 seconds sometimes.) Turn your flash light on poining away from the mirror and then hold it on the light sensor on the mirror side. It should activate the motor and the dimmer LED should turn on. If not, play with the dial pot (keep the flashlight on the sensor) until it does. After it’s in dim mode turn the flashlight away (or off) and wait to see if it goes back to day light mode. Play with the pot AFTER waiting. Then try turning the lights on, waiting again to get the mirror to return to daylight mode. Play with the pot again if it still doesn’t work. If it still just sits there, try moving the flashlight between both light sensors. If it didn’t go to daylight mode and you waited over 15 seconds then keep playing with it until it does. The light sensors are supposed to return the mirror to daylight mode when they both have approximately equal light. Remember the “10” second delay when going to daylight mode.
6.) If you made it to this step and it doesn’t work, well it’s 2:30 am and I’m really tired. You could probably tell anyways. If it does work then verify that the reverse override works by activating to dim mode and then connecting a jumper from the reverse over ride post to positive. (I actually didn’t check this on the mechanical one yet.)
7.) Oh, and when you reassemble it, turn the dial pot and the dial itself all the way over to one side and then check it before it’s assembled completely. I had a “special” strip of electrical tape that ran along the bottom of the board as well. I took a piece of wax paper, attached some electrical tape to it, then the old one on top of that and cut out the shape onto the new piece. I again cleaned the board with alcohol and attached the new piece to it. It worked out really well. I also freshened up some of the solder joints and cleaned the flux off with… alcohol.




Motor running continuously:
If the motor continuously runs, first (quickly) try to determine if the drive train is slipping. Do the belts bow out on one side with the motor running? Try a different size then. Can you push on the side of the output pulley with the motor running without the belts slipping? Try to see if the contacts are being depressed by the cams. Try pushing down on the contacts to stop the motor without getting caught up in the belts (I don’t know if you can actually, and I think one is for dim and the other for day light- haven’t had to look into this and it’s difficult to figure out what the little bugger is trying to do anyways). If the drive train is solid (not slipping) and the cams are pushing down on the contacts then I would check the contacts again. Again much of this can be checked by hand. You could try a continuity test on the top and bottom of the contacts if these seem to be an issue but I think the bottoms are hard to get to. Make sure the contacts aren’t bent and that the cams activate them.