Hey Joel,
On my 71 142 I installed a roofing product from Lowes on the backseat floors. It is a roll of aluminum backed with a black sealer. The thicker the better, It looks all nice and shiney. Available and cheap as well. Just cut to size with scissors.
Also I plan to put up a styrofoam panel behind the back seat in the trunk. I am doing up the trunk fancy and will paint the styrofoam black and cover with a thin plexiglass. I think I have a lead on some new trunk cardboard panels from Swedin, and wanted a nice clean look in the trunk.
Do not to forget to seal the interior of the door underneath the panel with a plastic sheet. That helps with the sound deading and protects the interior panels.
CVI automotive in Swedin has some Butyl putty that comes in a 20 ft roll used to seal up various interior surfaces underneath the interior panels.
I wish you luck with your interior project.
I am almost ready to reinstall a dash and rework all the dash intrumentation with a recently acquired ralley cluster dash. I also replaced the heater motor while the dash was out. It is almost simpler to remove the whole dash to replace the heater motor, than to do it with the dash in place.I hope to post some picks of the project when comlete. Featured is the new leather stiching I had put in.
Anybody need any nice gray Austrialian sheep skin seat covers that will fit a 142?

Joel
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Antique Swedish Steel 71 142E color V#102