Is the squeak on the right or the left?
Does it squeak with an up and down motion and no bumps, or just certain sharp bumps?
Can you replicate the squeak by pushing down in the trunk area and bouncing the body?
Have you removed the spare tire and jack and is the squeak still there?
Do you have a Shaker box in the trunk?
Squeaks are usually a process of elimination.
I had separate squeaks in my rear passenger side area.
The first squeak was an up and down motion I could replicate by pushing down in the trunk area with the car stationary in my garage. I used silicone spray on everything I could find under the car. I lubed all the shock bushings, every piece of rubber or urethane bushing I could find, as well as the rubber mounts for the exhaust. The problem was solved. The up down suspension travel sounding squeak was gone.
I never did figure out exactly what made the noise, by lubing everything it went away.
But there was a second/third squeak which took me a year to find. I was sure it was the spare tire, but it wasn't. That squeak only happened on certain bumps.
I lubed every hinge I could find, on the trunk lid, the folding rear seat hinges, everything.
It turned out to eventually be the Shaker 500 box. I dismounted it, silicone sprayed the mounting bracket bolts and reinstalled. Part of the problem was solved in that the squeak changed pitched. Then I lubed the trunk seal and tightened the red plastic nuts that hold the speakers to the rear deck lid.
Finally, all was solved.
I think I burned through a can and a half of silicone lube spray before everything was done and had spent hours tracking everything down. And it took a year of persistence to get things right.
2012 Shelby American Mustang GTS 580rwhp 485 ft/lbs Grabber Blue White Stripes
Ford Whipple, 5.0L, Tremec XL Magnum, MGW, Centerforce DYAD clutch, Boss 302 Torsen w/4:10s, 302R EPAS, Eibach Coil Overs, Fays 2 Watts Link, Dynotech DS, Shelby UCAs/LCAs, Wilwood brakes 6 piston 4 piston, Drake Bump Steer, heavy duty cooling, welded axle tubes, Interior Package, GT4 wheels, Borla Touring axlebacks, GT500 rear valence w/parking sensors, and airbag cover signed by Carroll Shelby.