I'll go with the switch as toxic was saying.
You could try cleaning the switch contacts. The switch can be pried apart and reassembled if you do it carefully.
As a second possibility, the circuitry in the switch itself could be damaged. According to the '91 CRX owners manual, the defroster is designed to shut off automaticly after 25 minutes. Given the switch wiring (power, ground, switched output and 2 lighting wires) and the way the switch acts (resets itself to off when power is removed) the timer is most likely built into the switch. I use a '88-89 defroster switch for my foglights and they don't shut off after 25 minutes, so scratch that idea if you have the older switch.