Left side speakers not working
 

A while ago my left speakers in my 2001 Audi allroad just quit (Audi symphony radio, CD, cassette) . About 6 months later they suddenly worked again. Now they work most of the time when I am parked but when I am driving they make a terrible crackling static sound.
When I rock the car the speakers come on and off. I have replaced the big front speaker and wiggled and checked the wires all the way from the speaker to where they enter the frame.
I suspect a bad connection but where? I have also swapped out the radio and amplifier - no change.
In old posts I read about too much dielectric grease somewhere but I don't know where.
What could cause this mysterious problem? Any thoughts would be appreciated.
Thanks
Audi Fahrer
Vorsprung durch Technik

P.S. I toured the Audi factory in Ingolstadt and it was amazing. 75% of all cars in Ingolstadt are Audis- really cool.

I suppose I should have mentioned that I found the problem. I took all the doors apart, checked wires, switched the amplifier, tried new speakers, followed the wires under the dash. Then, by sheer coincidence, I realized that, if I stomp my foot while sitting in the driver's seat, the left speakers comes on again. So I dug under the driver's carpet and that is where I found the problem. There is box under the driver's feet and a mess-load of wires go into that box, and apparently the speaker wires as well. I wiggled some wires and that fixed the problem for a while. I also put some Styrofoam pieces under the wire harness because propping them up solved the problem. I guess when it warms up here I might take the seat out and really investigate. Thus far I only reached under the carpet and felt around.
Hope you find the problem, it took me weeks of digging . . . . but I figure, I have a Bose system, I want to enjoy it!