Tom,
I've replaced one on my convertible years ago. There is one nut that is impossible to get at without removing the inner fender. The Buick Service Bulletin's method is to drill a 3/4" hole in the inner fender to gain access to that nut. That's what I did and now I wish that I'd taken the long road and removed the inner fender instead. Here's the Service Bulletins that explain the procedure. Don't forget to put a rubber plug back in the hole if you choose this method. Sorry, but my scanner is out of commission, so I had to take photos. I hope that you can read them. I wonder if a long handle ratcheting wrench would work? Good luck.
John
