I have this weird bad smell only when I turn the front defroster on. If I have the heat on any other vent I don’t really smell anything bad but if I switch to defrost it smells. I changed the cabin air filter and tried disinfecting the system through the fresh air vents as much as possible with no results.

Im hoping it’s not a heater core leak as I don’t have any issues with heating. But can a small enough leak cause smells only through defrost?

 

Your compressor kicks on for defrost, and not the other modes of heat, so I would start there.

 

The smell isn’t like a mechanical burning type smell. It’s more of like a vinegary type smell hard to describe so I’m not sure a fault of some kind in the compressor would ever make for a smell like it.

 

Yes.

 

Is there a way to diagnose a leaky heater core? Wouldn’t the smell issue be present when the heat is on in other vent modes since coolant is circulating through the heater core then?

 

Check the carpet on the passenger side and see if its wet.

 

Probably mold/mildew on the evaporator.
lysol helps for a bit. Sprayed into intake vents at windshield cowl.