I have a used OEM amp that is a pull-out from a car in Florida. It's yours for the UPS charges.

You can take the time to measure resistance in the ground, but a more effective test would be to start the car and measure the voltage to ground at the pins on the amp that Zip mentioned. ANY voltage over maybe 1/2 volt would indicate a bad ground. I'd bet you $20 the ground is fine, that ground anchor point has never been known to be a problem. And if that ground lug was loose, the fuel pump and BCM would be failing and you'd KNOW it.

Water damage to the amp, due to water intrusion caused by a leaf-clogged drain near the windshield, is quite common. But just by yanking the connectors off the amp, you should be able to see the severe corrosion that water causes down there.

Given that you don't know anything more than "it didn't work when I bought the car", it may be the head or the amp. (Obviously anything the "car audio shop" told you is worthless.)

I still suggest you start over. For less than $250 you can buy a nice, new head unit and after market amplifier. It is not hard to access the path from the back of the head to the amplifier area and you can run new wires from head to amp if you want to keep it simple. THEN, using the diagrams Zip mentioned, just cut the connectors off the cables going to the speakers and wire your existing (new) speakers to the amp. I'll bet you will have a better system than factory. In fact, I will guarantee it!