Appreciate the update.
I did eventually figure this out after removing the RHS rear panel - to fix the other rear window which was sticking on the way up.
Welded supports to the rear window brackets - to prevent flexing at the point where the regulator arm connects.
Got the RHS back together and the window works great. However, I lost a flat spring clip - used to secure the regulator arm to the window bracket - on the LHS.
The clip slots into a groove on this spindle (see red circle area in pic) of the window bracket. Darn
Not to sure this will be available at the local hardware store. Any ideas on where to get a replacement?
Cheers!