Thumping noise in speakers?? Short??

Yesterday my ABS light came on out of the blue (brakes are fine), and now if I have the heater going, and I turn on the rear defrost, wipers, or power windows, I hear a thumping sound coming from the back speakers???? Is this a short of some kind, and if so, what should I look for?? Thanks in advance!!

My 94 Corsica does that when there is not enough power going to the radio (low battery, heavy load, etc).