Go here,
FIAT ePER :: The FIAT Forum
Open it with Explorer and not MOzilla or Chrome.
Select SX in the first screen and then select 156 on the next screen and put in your chassis (VIN) number. The correct catalogue for YOUR car will come up.
Then browse to the door and glass sections and have a hunt for the correct part.
If you need help, let me know, I will look it up for you.
156's have some wacky differences in the different models,. Mossawi is correct that the facelift ones had some changes, but the facelifts only came out in 2002 if I recall...
Good luck