Hey Black99SVT, did you ever fix this issue?
I'm actually having a nearly identical issue with my GT as well. This past year as I have started to drive with the windows down again, I noticed the same "squeak, squeak, squeak" varying with speed. I've got no idea how long it's been doing this for and I really only hear it with the windows down when I am near something that can bounce the sound back to me. At this point I am assuming it's the brakes since I noticed that the sound goes away when the e-brake is applied.
I actually had assumed it was coming from the left rear since that is typically where I thought I was hearing the noise from. However I took both wheels off today and noticed that the right rear caliper will actually move when the brakes aren't applied. The mounting bracket is tight however so it's the caliper itself that moves in relationship to the mounting bracket and the pads. And the movement I am describing definitely isn't movement along the travel of the slide pins. I think the squeak is coming from the clips that are on top of the pad rubbing against the caliper. When the brake is applied the caliper won't move and the squeak stops.
I tried tightening all the bolts and the caliper still moves. I even replaced the slide pins since I had a spare set in the garage but that didn't make a difference either. Is it possible that the caliper is slightly deformed causing it to move? I do remember when I did a brake job last year that I had an awful time trying to line up the slide pins and bolts on that side. Has anyone ever had a similar issue? Is my best option to replace the caliper and bracket?