Adding mirror with compass option ...

to my 1991 caprice which already has the map light mirror option. I thought I could just plug in the new mirror and go, but it doesn't seem to work that way. Actually it wasn't a new mirror, it was from the wrecker off a 91 caprice, but it looks to be in excellent condition. Wiring harnesses for both mirrors have four wires going to the plug. When I hold down the button for three seconds on the compass mirror "CAL" is not displayed as instructed in the owners manual. Do I need to activate something in the computer to get this to work, or have I missed something? Any help would be appreciated. Thanks. Tom