IndyJimW

I believe it would be from the donor vehicle. The 1987 Chevrolet Monte Carlo. Good luck Jim Wood

If your Avanti is like my 1987 Avanti it has several hot wires supplying the interior. I have seen problems with these wires having loose contacts at the firewall plug. I would start there. Good luck, Jim Wood

You could also try one of the fuel tank cleaning and sealer kits. Amazon has one by KBS Coatings