I bought an instrument cluster off of ebay and it arrived with a cracked "glass" (it's actually plastic of course) due to poor packaging. I decided to see if I could repair the cracked piece - it was only cracked on one side. First I taped up the good side to protect it from getting cracked or scratched. Then I dremeled the clear plastic off on the one side, and then ground away the remaining clear plastic about 3mm beyond the opening of the cluster housing. Then I used a thick piece of paper and made a template for the replacement clear plastic. I purchased a small sheet of 3mm thick clear plastic for about $5 from a local plastics supplier, and using the template and my dremel, cut the replacement piece. Then I used clear silicon all around the outside of the black cluster housing where the clear plastic would sit, and pressed the clear plastic replacement piece into place. Looks as good as new from the outside. Total cost $5, total time about 2 hours.
