93 gm safari electrical troubles.

I have a '93 gmc Safarithat has developed a couple of electrical problems. 1. Periodically the battery just goes dead overnight, the doors are properly closed and there is no detectable electric usage.
any ideas?
2. both brake lights are not working, i don't know much about ele4ctricity but I did check the bulbs and the fuse (15 amp) all are good. I changed the pedal switch and still no change. Please help, we need the use of this vehicle!!
3. The cruise control doesn't work and I don't know where to begin with that, sio if you have any ideas keep the term9inology explicit and simple.
Thanks, jac

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