crazy electrical problems with 90 caprice

[oldskool90]

i have the strangest electrical problem i have ever seen, and it has been getting progressively worse so i need some help/suggestions:
it only seems to be affecting my rear brake lights,3rd brake light, signal lights, reverse lights, dash lights, and rear defrost.
basically (right now) none of my rear brake lights work, none of the signal lights work (front,back,left&right), my reverse lights are on all the time, my dash lights only come on when i turn my rear defrost on or i hit the brakes.
I've checked all of the bulbs & fuses but no problems found, which makes me think its a ground problem but i don't have a wiring diagram showing ground locations for the car. any help would be greatly appreciated.
thanks