91 Trip Monitor

I rtecently had a new stereo installed in my 91 convertible.  When I got it back, the trip monitor readout in the display does not respond to input on the buttons, and reads only average fuel mileage.  Every 30 seconds or so, it goes balnk and tries to display the trip odometer reading, then goes back to average mileage.  Is there any sort of reset procedure, or do I need to take it back to the folks that messed it up and hope they can figure out which wire they pulled out?  Any experience with this sort of problem would be appreciated.