I think by design the window regulator/rail/wire etc can not be replaced via parts but whole.
Its a simple job, which you have discovered requires taking of the door card, releasing the window from the regulator, remove the entire regulator assembly then go back from there with the installation of the new on.
This happend to me on my 201 and it was raining like crazy that day so I stop by the dealer, bought the part and replaced it right there on the curb parking.
Price for the regulator is as follow
202 730 13 46 Left : Retail : 171 Whole: 137 Dealer 103
for the right side sub the 13 for a 14.
Best of luck.