Please help diagnose starter issue - won't turn. Car starts from a roll just fine.

[clydesdale]

Please advise:
We have 12v power going to starter solenoid. When we key to start, the other side of solenoid gets about 7v of power. The car was starting slow for a day or two and the stopped. I tried to run power directly from the power terminals on the passenger firewall to the starter using a large stereo power cable, as I did not see a way to clamp as standard jumper cable on there. It was 10 degrees outside though, so my fingers and brain nearly stopped working. Any idea how I can determine that the starter is good or bad?
Jumping from the battery does not help either. Thanks for your help.

[ForcedFirebird]

You can bump the starter using the round diagnostic plug. If you peel the boot back to expose the wire colors, black/yellow is starter engage and red is battery power. If the starter still doesn't go, then it might be past it's time.

[tomstin]

Check the engine grounds. Bench test the starter.

[clydesdale]

Thanks, I will look at the wiring again. I am trying to diagnose this in the car. I replaced the ground from the oil pan to the frame last year due to the same issue. That ground seems to be fine. Which other ones are most suspect? Also, is there a way to bypass the grounds. We attached a jumper cable from the engine to the shock tower to simulate another ground.
So, with regards to the wiring, do I try to put power to the black and yellow wire lug?