I recovered my firewall back panel with vinyl from a local fabric shop. Just staple in place over the factory carpet. NO GLUE. Turned out great and was simple and cheap.
Here is my build thread. It's towards the bottom of the page:
//www.fiero.nl/forum/Forum2/HTML/137108.html------------------
'87 GT - 5spd, Project Car- 3800SC II: Full Rebuild, VS Cam, 90# Springs, 3.3 pulley, TOG headers, 3.5" CAI, A104's, 180* thermostat, 36lb injectors & SPEC 3+ clutch- 38SP3CL
'86 SE 2m4: my first car at 16- SOLD 
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So now you'd better stop and rebuild all your ruins,
For peace and trust can win the day despite of all your losing