There's actually an on/off button for the temp/compass display on the mirror. It is the button located on the left front of the mirror itself. Push this and the display should turn on.
There is no reset button on the thermometer in a rear view mirror on a 1999 GM Yukon. However, you can try to reset it by disconnecting for five minutes.
http://mojavetrail.webng.com/AutoDimMirror.pdf
Your automatic dimming rear view mirror may not be working as well. If so, you have a blown fuse. A 5 amp fuse I believe.
your onwners manual will tell you how to reprogram the compass
That might be hard to answer. The vans never came with the compass in the mirror so its probably part of the conversion package. usually the mirror is tapped into the lighted visor circuit. Does anything else not work?
Mine is a small box attached to the rear view mirror
It is called a compass module. It is the part that reads compass headings for the cluster display.
Have you checked your liftgate fuse? In my 2006 Yukon, when the liftgate fuse went out the rear ac wouldn't work nor would the electric locks for the liftgate itself.
On my '97 Tahoe the owners manual has detailed instructions on how to reset the compass. This of course will only work with a compass mirror that has power. So does the mirror display directions but just incorrect or is there no display indicating that there is no power? The former can be cleared by selecting the region and driving around in circles a few times; the latter will require a little wire chasing or that you have a defective mirror module.
there should be a screw on the mounting plate for your mirror. most of then are a hex head bolt the just holds the mirror to the mounting plate. look at where the mirror connects to the windshield, that's where the bolt should be.
yes it should be plug and play
Rear view mirror.