How much to replace motor for convertible top

Hello, Yes, diagnostics can definitely identify a bad motor. There is no reason for guessing about it. I've only been fooled by one motor, and that was a motor that was grounding internally when I was about 15 years old... it's just not rocket science. As for the cost, you will do well to see an independent Audi/VW/Porsche or European specialty shop. Don't pay dealer price on a 10 year old car, and do get more than one quote/opinion.