I remove the backing from the front cap and side skirts by laying them out on some heavy plywood and carefully scraping with standard paint scrapers. Soaking the wood over night using wet rags with a sheet of plastic over it to prevent it from drying helped to soften the wood that was left.
After getting most of the wood and cardboard off, I used Acetone to help dissolve some of the adhesive. (Not knowing the composition of your siding, you probably need to try the Acetone where it doesn't show to verify it doesn't affect your siding.) Then used a sander to remove more. I didn't remove every bit of the glue, just enough to allow the fiberglass to adhere smoothly to new plywood. Not positive but I think the original adhesive was polyurethane. I did use polyurethane to put it back together with.
On the right front side wall I had about 24" of de-lamination back to the cab over window. I removed all the loose plys back to good wood. The fiberglass still had a single ply adhered to it which I dried and then re-laminated with new 1/8" plywood.
It worked well and turned out to look great. It was a big job which took a lot of scraping and sanding but was well worth the effort.
Together, the wife and I spent probably 2 full days prepping the glass before glueing it back on.
Good luck on the project.
