Mine did it last year. Since moving the back upright seat part upwards as I sit almost upright as oppose to the 'cruzin' bad boy style. The squeaking has completely gone
It's usually the left bolster that squeaks.
I also have a bad back so I've no choice but to sit as straight as possible.
You're not alone mate! In my case I tried raising and lowering the seatbelt djuster to no avail, I still get the "squeaking where the seat belt rubs the top of the seat when in use" all the time grrr... maybe some kind of grease will fix it, but not permanently?!?