I had the same thing happen to my 91, I would have to press the remote against the glass and *pray* it worked. If there seemed to be any nearby RF interference, I had to use the old fashioned key to unlock the door.
New batteries solved nothing.
Casual examination of the system innards showed no obvious problem, messing with connectors or the antenna did nothing useful either.
Purchase of a completely new keyless entry system DID solve the problem (about 4-5 years ago), but they are not available anymore. I retained the original wiring harness, so that was unlikely to be the problem too.
I recommend that you contact Kenwood and see if they can help.
[EDIT]: Kenwood was the OEM for the keyless entry.
On second thought, I would ship the unit to BrianK and have him work his magic on it...caps dry out and there is probably one in the RF loop.
Drew