My power antenna failed this summer. After taking it apart, I found it was simply a broken belt, which I thought should be an easy fix. Unfortunately, I had a hell of a time finding the belt, so I thought I would post a source. The belt on my '91 318i is a 2.03mm (MXL) pitch, 60 tooth, 6mm wide belt. I found a supplier online who has a decent online store and will sell you a single belt for about $3.50 + shipping. I'm in Alaska, so shipping was more than the cost of the belt, but still much cheaper than replacing the whole thing(and a lot less wasteful). They have various other options for materials and tensioning material, I went with a Urethane belt with kevlar tensioning (A 6B16M060060).
https://sdp-si.com/eStore/