I have an air vent that smells like something dead or rotten seweror garbage, worse after the heat is turned on and runs. It is so bad I have had to shut the vent completley closed.

Its time to do a manual eyeball inspection

Sounds like you may have a dead rodent in the ductwork. Look for chewed ductwork.

If the smell is only from one vent you got it localized to one area. Inspect the duct going to that room, you made need to check below the floor in the area and above the ceiling for dead animals.