Signed up to post this in hopes it might help someone.
Disclaimer : I am not responsible for any damage you do to either yourself or your property by attempting this repair. If you do not feel comfortable you can purchase a used mirror for $60-130 on Ebay.
Symptoms : Compass display no longer lights up, is or dim.
Potential Cause : Cold solder joints with resistors on circuit board.
I almost purchased a replacement mirror off Ebay but decided to crack mine open first to see if it would be a simple repair. Researching NASIOC there must be a rash of these mirrors with the same symptoms. I was able to isolate the fault in mine, which is probably the result of thermal expansion from the resistor, slowly over time breaking the solder connections.
First start by unplugging the bus from the back of the mirror.
Then use a old credit card or a plastic guitar pick / spudger to work your way around the bezel of the mirror releasing the plastic snaps which hold the front bezel to the rear backing.
Be gentle and be sure to release all the clips, do not try to rip the front bezel off before you have worked your way all around. One each end of the mirror there are two large clips you will have to release. They are the hardest ones.
Once the backing is off remove the connector for the auto dimming mirror.
Next you will need to remove the circuit board from the back of the mirror chassis. Be gentle with the board and note the clips. Start with the clips on the left as highlighted.
Once you have removed the circuit board it should look something like this.
In my case I have highlighted the particular resistors causing issues. You may be able to confirm this while pushing on the back of them with a piece of plastic while your bus is still connected to the circuit board, and observing the behavior of the display.
Here we are after touching up all the solder connections, using flux to ensure we got a good even joint. Be sure to clean the flux off with alcohol and cue tips once done. Use the lowest temperature gun/setting possible and expose the components to heat the least amount of time possible to do the job. With excessive heat/exposure you risk damaging the resistor.
Now test before re-assembly, following directions in reverse.
This may apply to other models/years with the same issue. One thing to note was this mirrors circuit board is probably used on other models because the display had a temperature read out which is hidden and disabled on my mirror.