There are only two places that (should) let air into the "cab" from the outside--the cowling These are an obvious round hole in each corner on the "roof" of the cowling. Right side connects with the heater, and left has a dedicated fresh air vent with a large obvous door down by your shin. Ditto for the heater, that vent is built into the heater
Fumes, I would check the entire pass compartment for leaks. If you have created entrances into the body for various things start there. Missing seat belt/ seat mount bolts, missing / torn shift boot, unplugged holes in the firewall AND LEAKS INTO THE TRUNK. This includes a missing/ torn trunk lid gasket, the fuel filler gasket, any other holes from runs up into the trunk
As you travel down the road, the rear of the car vacuumes air/ exhaust fumes up into the rear area around the car. This is why on a dirty/ snowy road, the rear bumper/ deck gets dirty. This same process sucks fumes into holes in the trunk
Try putting a bright bare lamp bulb in various places in and under the car, then look "through" the car for shining light
Photo I found on the 'net shows the cowl vent holes
View attachment 1715695436
Driver side vent box if yours is missing
View attachment 1715695438
