Rear defroster

I can help with the battery light. If the charging system is not recharging the battery while driving, the low system voltage is going to affect other computers of which your car has many. Start by using an inexpensive digital voltmeter to measure the battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. If it is low, the problem can be further narrowed down by measuring the voltage on the two smaller wires on the back of the alternator, but those readings MUST be taken while the engine is running.

Sorry for the delay. High winds, trees down, power out, 'puter dead until a few minutes ago.

What relay are you referring to? There's two fuse boxes. One inside on the left side of the dash and one under the hood. I don't think you have a relay problem. After all, it appears the defroster is trying to work. Lets handle the charging system problem first, then see if that solves the intermittent defroster. The system is run by the Body computer and it's going to get confused if system voltage is low.

If the problem persists, suspect the Body Computer. There isn't much inside the switch that can cause a problem.

I think you might be due for a load test on the charging system. Besides voltage, (electrical pressure), which is what is detected to determine when to turn on the warning light, the alternator must be able to provide the necessary current, (volume or flow). There have been a lot of problem with wear of the brushes inside the alternator that leads to it being dead and the warning light turns on. That always starts out being intermittent and can stay that way for weeks. The part is very inexpensive and fairly easy to replace, but most people just buy a rebuilt alternator.

A less-common problem is one of the six internal diodes shorts. That will show up on a load test as it can only deliver very close to one third of the rated output. A 90 amp alternator is one of the common sizes. If it has one bad diode, all it will be able to produce is around 30 amps. It takes around 10 amps to run the fuel pump, 10 amps to run a pair of head lights, then you still have the tail lights, radio, heater fan, fuel injection and ignition systems, so there isn't any spare capacity left over. The rear defroster also only draws around 10 amps so that is insignificant, ... IF the alternator is working properly. You might have stumbled onto a clue that there is a charging problem before something worse happens.

Very happy to hear it's working. Gonna have to add that to the memory banks. Where is that relay located?