I had a stereo fitted in my car about 2 years ago and at a similar time of the install the car developed a squeak everytime I went round a corner...
It really did my head in, I played around with the unit but still couldnt get to the bottom of it so I took it back to the company that fitted it and said it was driving me mad. The fitter came in the car with me and it baffled him too, in the end we found the problem, it was nothing to do with the stereo it was the seat that was squeaking under the pressure of my fat arse everytime I went round a corner.... bit of WD40 on the rails and its been fine ever since
Was rather embarrassing though .... not been back there since