Been there, tried that. Grounding one speaker separately in the back did not work -- no rear speakers at all.  I ended up running another wire to the front -- bit of a PITA but it works.  There was talk of using a 'floating ground adapter' or something like that not long ago.  The users of said widget seemed to be happy with them.

Chuck
'85 Wolfsburg Westy - 'Roland the Road Buffalo'

In a message dated Wed, 15 Aug 2001  8:27:37 AM Eastern Daylight Time, Automatic digest processor <LISTSERV@GERRY.VANAGON.COM> writes:
> Date:    Wed, 15 Aug 2001 02:36:48 -0600
> From:    Rachel Cogent <Gnarlodious@EARTHLINK.NET>
> Subject: The rear speaker ground.
>
> All this talk of car stereos has me wondering if there is a shortcut to
> getting two separate grounds for the rear speakers. Could I use the frame
> for one and the ground wire for the other? Do I need to pull another wire
> through the tube?