In your 1990 300zx when you press the brake pedal or turn on the headlights the power drain is so high it shuts off other components like the radio what can be the problem?

Well from what you have said it sounds like low output from your alternator, but that would also discharge the battery pretty quick, I have also seen batteries so bad that they wont put out 12 volts, but again you would not be able to start the car either. So that leaves a grounding situation in the lighting circuit and that should have blown a fuse right away. I dont know what has been done to this car so diagnosis is difficult but i hope i have given you someplace to start. just so that every body knows, the problem turned out to be the positive battery cable. on the surface it looked fine but once i peeled back the insulation it was solid white with corrosion for abouut 3-4".