When I need to remove something from a plastic window (or other plastic
part) that won't come off with plastic cleaner or soap and water, I use some
kind of PLASTIC SAFE aerosol cleaner.  Typically these are electrical
contact cleaners. USE CAUTION and be sure to use a quality material, check
the label carefully to ensure it's plastic safe.

There are many manufacturers. LPS is available from Aircraft Spruce if you
can't find something locally.

http://www.aircraftspruce.com/catalog/cspages/lpssafeclnr.php

Ted Stanley