return line location

That line (By the horn, right?) looks like it attaches to the charcoal/vapor canister, but it also looks like that car didn't come equipped with one. I owned a 72 Dart Swinger & it did not have one from the factory either. I have a 74 Duster (parts car) that DOES have/had one & that line was held to the frame rail with metal clamps that screwed into the frame rail itself. I believe there were 2-3 between the canister & firewall before heading further rearward. Yours also does not appear to have the screw holes in the frame rail either. I will take pictures of mine while it's still intact, if you want.