Ive got the same problem.Your motor chatters when trying to lift the roof right?
The only solution is to replace the whole motor with a used 1 or have it overhauled by SIMPLY SAAB from Bristol (see ebay-tonneau motor).Used motor around £125.Re-con £300.Once refitted it WILL have to then be taken to saab for re-timing.This MUST be done or you will damage your car...Trust me on this 1 fella!
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Try fixfault.com to fix cars.