What should I coat the entire rear/underside of the plastic dash pieces to protect/reinforce it?

 

The dash on my '02 was really bad. I bought a dash cap and glued the snot out of it. That held what was left together and looked like a new dash. If you have the dash out and want to reinforce it, make like a ground squirrel and gopher it. I think the cap was about $160 or so on Fleabay.

 

A good epoxy and some dry wall tape. The open mesh. Make a wide V on any cracks and trim the tape to fill the groove. Then put a layer or two over top.