Window slippage

The driver's side rear window has a bad habit of falling down a little, especially on bumpy roads, resulting in a that little whistle-whine in my ear.
Pushing the up button on the console fixes it but it always comes down again...
Regulator or something else?
I do have experience replacing the regulator on the driver's front door but I understand the rears are a little more inaccessible?
-shawn