I had the same problem with my 2000 SSEI left rear window. I found a new regulator with motor for less than $50.00 (delivered) on ebay.
Here is the link PONTIAC BONNEVILLE WINDOW REGULATOR&MOTOR 00-05 REAR L (#330351649876)
OEM # 10387939
It was all new and was a direct bolt-in replacement, it works as good as new.
I see this vendor also has a pair of front regulators for about $91 shipped. Item #330352153330
I have no relationship with this vendor
other than buying a left rear regulator and being very happy with the cost and quality. They have over 40,000 feedbacks. YMMV.