I have some direct experience with OpenSky, having replaced an entire unit courtesy of A2Steve having one at the right price at the right time (albeit on the other side of the country, but who doesn't enjoy a weekend in the Brecons!). My old one was doing what yours sounded like it was doing and having removed the whole unit, I tried to free it off. Nothing I could do would make the right hand side start moving. The motor (when plugged in) was just grinding against the cable, which, the more it happened, the more it stripped the gears.
A2 Cars offered to refurbish it for a lot of money, so I ended up buying A2Steve's working unit and fitting it. I then got A2Cars to fit their specially designed pickup bracket (much cheaper than a full refurb) and have cleaned and lubricated the whole thing. The pickup bracket is made of billet aluminium and replaces the riveted pewter Audi effort, which is prone to wear in the rivets: this causes the motor to slip on the cables at the first sign of any friction.
Whilst it is an enormous faff taking the headlining down and swapping the roof out, it only takes an afternoon/evening. You can then clean all the seals and lubricate with the Audi silicone paste/spray and get the A2Cars mod done when time/funds permit.
Sorry for the ramble, but I have ended up with a nicely working OpenSky and have done it in stages to spread the cost.
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