Alright. This has been driving me nuts. My left rear corner of my car is squeaky. The car is on H&R springs, Illumina shocks, and RSB and Motivational Strut mounts. It just happened recently. I've had all these mods for a couple of months now. I don't know why it's starting to squeak now. It may not be due to the mods. I didn't use the red Motivational mounts.

 

Considering its coming from the suspension squeaking usually means metal to metal rubbing.
Does it only happen over bumps?
There is a write-up in the How-To's to fix your problem.

 

Take your rear suspension apart and make sure your shock piston isnt rubbing against the side of the hole in the motivational mount.

 

I've dealt with this before. It happened on my 97 with AGX/Maxspeed/RSB and my friend's 99 with AGX/B&G/RSB. The noise for us was NOT in the suspension. It was in the tack welded area above the strut tower. It's been a while so I don't remember the details. But I ended up using a large flathead screw driver and prying two pieces of metal apart by the tack weld. Get inside your trunk and have someone bounce the car. You will get a general idea of where to start prying. After you pry spray some silicon lube in there and be happy. Most likely you have the same problem. It's not coincidence that its the left rear for your car also.