My 1991 Chevy cavalier won't start

Two months back I purchased 2 cavaliers without seeing them (big
mistake), even though I didn't pay much, I'm starting to have my
doubts.
The original idea was to get a car for cheep and fix it up while
learning car repair. I think I got in over my head. One had got into
an accident and was going to be the parts car, and the other needed an
alternator and muffler. One had only " a little rust ".
After I got a good look at them it was obvious that it wasn't worth
it, my attention went to the banged-up one because, sadly, it was in
better shape. Unlike the other one there was no rust and great
suspension and all the rubber is good.

The problem is that I can't get the car started. The car had been
taken apart and everything was disconnected (the AC and alternator were
also gone). I think I connected all the parts and wires.
The cars lights, radio, wipers and heater fan all work. The battery
sits at 12 V and when you crank it makes no sound, except when you
listen under the hood you can hear the fuel injection. The starter
makes no sound. I tried testing the starter by using a voltmeter -
to S wire and + to battery and the voltmeter showed an unsteady voltage
of up to 9 V, but no higher than that in the run position. When put
into start position there was still no change in the voltage.
I know that the voltage at the S wire should be 12 when cranked and
zero when not. I think the problem is inconsistent voltage and voltage
in run position. Being new to car repair I don't know how to repair
the problem and more importantly I don't know what the problem is.
I also tried starting the car in drive, reverse, neutral, and having
the key in start position while moving the gearshift to test the
neutral safety switch.

I would greatly appreciate any help.