I have a 1991 BMW 525i when I turn on the defroster it seems to overload the electrical sytem?

You may want to bring your car in for a charging system test. A "normal" alternator puts out between 55-100amps depending on the car. Both the A/C and the rear defroster draw large amounts of current ( about 20-30 amps). so if your alternator is only able to produce, say, 50% of its rated amperage than the excess load from the A/C may be all it takes to start draining the battery. There are several reasons that an alternator may not be charging well. It could simply be a loose belt, or maybe a bad diode in the alternator. If you don't know much about cars, I recommend taking it in for a charging system test.

Wiring harnesses on this model are notorious. Generally a wire has frayed and is grounding the system. There are small electrical shops that specialize in these types of problems. Usually pretty pricey due to the hit and miss problem of a short. (sometime it is shorting sometimes not) But you have to address it or you will be out one night and end up being towed home. If your lights are just dimming when you turn on the A/C or D/F its not a big problem. Have an electrical specialist do a quick check. Alternator or generator might not be working at full capacity. Good luck.

The best way to deal with this is get about a pound or three of C4 or any other similar military grade plastic explosive, sit it in the back seat with a remote detonator and set it off in a very remote area. Film it for a laugh too. I probably should mention that I don't like BMW's so my answer may be a little reflective of this :D

(Is also humor, take it to the dealership!!)

They are on a common circuit worked from the a/c compressor. Check your fuses are correct for a/c system, also check to see if the compressor is under to much load. You will need a pressure gauge (Air conditioning technician) Over tight drive belts, or belts running out of alignment can cause this, plus bushes or bearing wear, thus causing heat friction is another.

The defroster and the A/C have one thing in common- the a/c compressor. It could be bad but on my vehicle when the compressor comes on my headlights momentarily dim. I guess this is normal.

Don't worry, soon your whole wiring harness will fry. My friend had a 525 and he went through 3 wiring harnesses. My advice- trade it or sell it or wreck it.

Your baby needs professional attention!