Something is draining my battery fast when the car is running.

What is going on with my car. I bought 2000 my z24 and it
had the stereo harness cut,  car ran fine until I asked
someone to hook up a aftermarket stereo and this is when
the problem began. Couldn't figure out the wiring so he
could only get 2 speakers to work front left and rear right.
The car had dim lights, and then the battery came on. So I
took the stereo out and capped the wires again. Still have
a short or lose ground somewhere? Car wont keep a
charge. I bought a new battery, alternator, fusable line,
ecm. Where are the grounds to check them? I'm at a loss
now. And getting broke. Do I need a new engine wiring
harness or a stereo wiring harness and hook the stock one
back up? Any one please help!