HU not grounded!!!

How would i ground my head-unit? I dont think its grounded, but it works! The reason i say this is because i had alternator whine, so an installer suggested  that i ground the HU seperately, heres where the problem comes in. When i removed the HU ground that was plugged into the harness and grounded it elsewhere the HU still had whine and when i accidently bumped the ground wire off the HU did'nt shut-off but still worked!?, so in that sense it was never grounded or its grounded somewhere i dont know about. So the HU can run even if its ground wire is not connected to a ground point. Anyone have info or suggestions? Its a pioneer deh-p860mp. Thanks!

It is most likely getting a ground from the antenna plug. If you unplug the antenna i bet the radio will shut off. Take your radio ground to some a metal part of the vehicle that grounded. If you have a good ground and there is no noise, then plug the antenna back in, if you get noise then, that mean the antenna is picking up the noise. The antenna base could be corroded and not making good contact with the chassis of vehicle. Jazzy