you grounded something in the front to something in the back? try to make grounding wires as short as possible - there several designated grounding points under the glove compartment with comb style connectors - use them. it will not completely eliminate the issue but will reduce the influence.
can you use shielded cable for the speaker's 1.5 foot line? try that too...

tell us if you get any improvement, if not, we will dig further...