I would dampen the rear of the cab. And do it before you lay on the fiberglass. Just be sure to put some foil on everything you will glass. I have done a few boxes with fiberglass rear panels but never one fully glass. I have picked up lots of good stereo info on
www.carsound.com and
www.termpro.com both have good fourms.
Heres a good tip for any place that has large flat runs (i.e the back of the cab). Lay down one or two layers of glass, then lightly glue in some strips of rope. Then glass over that. This gives you better strength. Fiberglass is superstrong when it is formed around bends. Large flats runs tend to be strong but flemsie. Also, use wood braces. So build the box alittle big and brace the heck out of it.
Any-hoo, look at the forums and you will have many, many ideas. I don't do this for a living or anything, I just have been into the stereo scene for a long time.
Chad