While the harnesses (factory and the wiring kit) had all those extra ground wires labeled for various things, my new head unit only had one ground - so none of that other stuff really mattered.

I grounded mine to the chassis - there's a bolt on each side in back of the opening where the HU slides in. I used my tester to "prove" the one on the right was a good ground and used that. Sandwiched the ground connecter around the bolt between the two brackets that the bolt holds together.
No noise at all, not even on AM.