2005 astro speaker question

Hello all,
I just got a 05 astro van and the driver speaker works for about the first
25 miles on the road and then dies out.
Before I open the panel would it be a radio ( stock ) problem or a speaker
problem.

ALSO

it is a cargo van, where would I find the rear speaker wires inside the
harness, would I have to remove the ceiling liner to locate it?, what colors
would they be.

Thanks for your help.

Dave-