I had what sounds like the same squeak and it drove me nuts. I eventually traced it to the speaker grills. There are plastic tabs the help hold the grill in place and they were squeaking as the rubbed on the dash material. The remedy above solved the problem.

__________________
LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold