77 bronco - won't start without parking brake set. Electrial

I have a 77 bronco with a strange electrical problem. (I think) I have
been ignoring this problem for months but now I can't.

1. the batter is draining
2. when I jump the car, it will only turn over if I set the e-brake.
3. when I release the e-brake all gauges, lights, and radio cut out.
sometimes they come back but less and less.

does anyone have any idea why the e-brake would have anything to do
with the electrial? I don't even know where to start. I've looked at
the brake cable and it doesn't seem to be pulling or rubbing on
anything.

Thanks,
Karen