1971 Duster exhaust smell in cabin after new muffler and tail pipe

Yup, two problems. One, that tailpipe is too short and a shoddy job. Second, you’ve got a hole somewhere- floor/trunk pan plug, trunk gasket, heck even tail light gasket or rust hole that’s letting exhaust in.

I’d absolutely take it back and tell them to fix the tailpipe because it’s causing exhaust to get into the cabin, which is a new problem related to their fix. After they sort out the tailpipe, give your plugs/gaskets/seals a once over and see if you can find the entrance point.