Since the common rear speaker ground wire retrofit is the fix then I
would like to chime in with:

-Since you are running new wire anyway then get some decent gauge wire
and replace it all. Better mechanically and for power/signal transmission.
- Consider running just low powered signal to the back (if your head
unit supports it) and installing a modest amp in the back.
- Make sure you have adequate chassis ground for the head unit, some are
more picky about this than others.

I like the design of running signal to an amp located near the speaker,
ideally each speaker would have it's own amp but I often compromise with
a dual channel for the rears and either a second dual channel for the
rears or just run them off the radio's amp directly.

Larger wire helps if you have a big ol' honking power hungry system but
it's also useful if you want something that will last a long time or you
want to upgrade later.

hth,
TBerk
Remember- Clarity & Power. Balance in all things.