'98 Corolla Electrical Problem

I am experiencing an electrical problem as follows: Frequently, the seat belt warning indicator flashes continuously despite the fact that the seat belt, driver side, is engaged. When the belt is unbuckled, the light goes off, the exact reverse of what one would expect. Also, when the the flashing condition is present, the light on the AC button will be glowing dimly when OFF and bright when ON. When the seat belt warning light is NOT flashing (operating correctly), the light on the AC button does NOT light whether in the ON or OFF position. Car has 79K on it and only recent changes were a new battery (precautionary swapout) and the replacement of the stock radio/tape deck with a CD player. Any help greatly appreciated. Am planning on delivering this nice car, in all other aspects, to my son in college in Florida. I do not want some monumental electrical problem to become a burden to a student!!! Looking forward to any ideas or similiar experiences.