I have observed quite a few forum member inquiries regarding whether or not to replace the entire power antenna motor or buy the nylon ribbon kit to replace it yourself which is a far more economical option and available at most of our C3 parts supplier partners - and, its a satisfying DIY repair in my personal experience - as well as maintaining the superiority of the OE motor which feels/appear and sounds far more robust and better designed than the stuff made by the nice peoples republic of communist China folks that are aiming a lot of missiles at us paid for by us. (just sayin') Following is a video I stumbled across. His GM vehicle that he pulls it out of is not a C3 and easily fast-forwarded; but, the antenna unit and the nylon ribbon replacement process shown in the video is identical. And, the fellow does an excellent job of talking through it at a nice pace if you've never done it before.
Hope this helps others who are contemplating their `repair vs. replace' options.