Gas fumes when heater on

I am not sure how the regulator will cause this because it is part of the pump in the fuel tank. However, the fuel pump is under the rear seat so I suspect that there is a vapor leak with this pump itself. If it is coming from the vents then this would mean there is a fuel leak under the hood. This is possible but again, the vents may be recirculating the smell but it is originating from the fuel tank under the seat.

I would remove the seat and see if the smell is strongest under the seat. This will tell you that it is coming from here.

Keep in mind, this is the most common cause of fuel smells inside the cabin on vehicles with access to the fuel pump under the rear seat.