common ground

To put a new head unit into a vehicle with a common ground setup, you need to run new wires to all the speakers.

When I was installing professionally, a guy brought his boat to me, which had a common ground setup and he asked me about upgrading the speakers.  I told him I could do it, but he needed to upgrade the deck also.  He said he was trying to save money and only wanted to upgrade the speakers.  I told him it would play for a little while and quit.  He didn't care.  A couple weeks went by and he came back to upgrade his deck and speakers because it had quit like I told him it would.