My car is in the shop right now for a squeak from the rear, however I diagnosed it for them. Hope it is the same for yours but this is what I did.
My squeak is coming from one of the bushings back there, I sprayed the pan hard bar bushings, the shock bushings and even the rubber for the exhaust hangers with some wd 40, noise went away for a few days but after the first rain it washed it out and it started back.
Hope this helps you, I should find out for sure tomorrow and I will let you know for sure what I found.
I only assume that with my car being a year old, I have only put 4500 miles on it, that it is because I don't drive her enough.
2013 ford mustang gt, black, bought 16th of August. Only mod so far is tinted windows.