Strong exhaust fumes when heat or A/C is on

Good morning,

Do you have any exhaust leaks in the system? A leaking exhaust will allow exhaust fumes into the car.

https://www.2carpros.com/articles/how-exhaust-systems-work

Are you sure it is not raw gas fumes? If it is, then you have an external fuel leak which must be addressed immediately. That is a big risk for a fire.

When you have the AC or heat on, it pulls in air from the outside and that is why you smell the fumes.

Roy