HELP - Factory Fitted Alarm Fob Probs

I’m the proud owner of a ’96 P Celica with a factory fitted alarm with a problem. All had been working fine with the remote etc when it suddenly stopped working (have been advised that it might have been “tampered” with by some local amateurs with a cheap signal grabber ?!). The alarm fob is working fine (have had it tested and is sending out a nice strong, clean signal) but the unit in the car just doesn’t respond. Have tried the manuals suggestion of holding both buttons for 5 seconds but with no effect. When the doors are shut, the dash LED comes on and the alarm appears to be arming itself. No one seems to be able to help so am booked in to a main dealer next week BUT can hardly afford their prices so any suggestions would be much appreciated ASAP. PS I’m not very technical !