Like many BMW convertible owners I have had major problems issues with my BMW (E36) convertible roof, i.e. won't open, wont close, stops halfway, etc, etc.
I found a garage called "Cayman Auto's" that specialise in convertible roof faults (all car manufacturers).
He charges £60 + VAT to diagnose the fault and then tells you what it will cost to repair/fix.
I spent £140 to get my non-working roof working again, which is a bargain considering how many times I've re-synched the motors, got out multi-meters and searched on-line for faults/cures.