I'm not an auto expert - I've had the same problem - but I have an '02 liberty. You don't mention whether the window slides freely (e.G. You can slide it up and down with your hands).
If you can, I suspect its the regulator. I had two of them (both rear) give up the ghost within 6 months of each other (one before the warranty was up and the other after). If that's the case, you'll have no choice but to replace the regulator. The price for the part alone is about $175. It is time consuming to replace to expect the dealer/garage mechanic to charge accordingly for labor. You could also do as I did - use internet resources and learn how to do the repair yourself - as I said its time consuming but not all that difficult.
Tuesday, September 22nd, 2009 AT 2:29 PM