Help with grounding head unit.

[wew lad 5,000+ posts]

also, just for shits and giggles, I measured resistance between the two halves of the initial ground from the back of the HU into the factory wiring that I cut into earlier, it measured -40. As well as checking between the wire coming out the back of the HU to the new ground wire I ran earlier. What's up with that? Good or bad?
Edit: If I swap the leads around, it reads 40.

do you have the black lead coming out of the factory ground grounded to the headunit wiring harness?

splice/solder into that. if that doesn't work add a second ground directly to the dash metal behind the radio, or even better directly to the firewall if you can. sand down the metal first. use a bolt if possible.

just to be ultra clear you need a ring terminal crimped onto the ground wire that is grounding to the dash metal or firewall.

you do not want to be grounded through the antenna so you're right there

 

[Shadowborg]

Sorry just noticed there was another reply. Yes I initially had, and currently have, the factory ground connected to the head unit harness ground. I had tried to run a new ground from a bolt in the driver's footwell to the head unit harness ground, noise was still there, so I just put it back like it was. I may try splicing in and having two grounds. But like I said before, the resistance reading between the head unit harness ground and factory ground was really high at 40ohms. The same reading between the head unit harness ground and the new wire I had run. I don't know why that is. I tried installing a ground loop isolator on one of my sets of rca's, it mostly got rid of the popping when I use my power seat controls, but the whine got WAY worse. So I took that off. Come to think of it, my amp itself makes a whining noise, maybe the rca's are just picking that up. Is it normal for amps to make a whining noise? This is my first semi serious stereo system. Another thing is the whine gets louder when I touch my brakes, for whatever reason. I think the car might also have a weak electrical system, even touching my brakes causes my voltmeter to go down.