I thought that the regulator was the spring thing and if that goes then the glass will slide down but I could well be wrong. I've had to change a winder mechanism in a RR and it was well fiddly but on a 90 it should be easier.
The glass in my door would fall down not slide but just drop as it fell off the bottom metal strip that holds it up. But I don't think that's your problem. It was tricky to fix as well!
Not much heklp am I?
Mike