I have a small exhaust leak which always leaks into the cabin when I'm at a stoplight using the heater or air condition. I want to know where does the heater airflow comes from. I want to see if I can redirect the air flow to source with fresher air. I know I could just fix the exhaust leak, but I'm wondering why the heater gets its air from an area next to the exhaust pipes.

Air comes from the Cowl. Thats what those big holes are in front of the windshield.

Thats why cowl induction scoops that are not sealed to the carb often result in heat entering the passenger compartment.

Try to get a good look at your blower box for damage. I had this problem awhile ago, and only discovered it when I had to remove the entire blower/a-c evaporator box and found a big hole punched in it near where the passenger exhaust manifold was. That, and my hood to cowl gasket was missing, which probably let some fumes in as well.

Thats probably my problem. I might of cracked the fiberglass when I was installing headers.

^^^This^^^

That material deteriorates REAL BAD right there anyway, even if it's never been physically touched, from the heat from the manifold; EVERY SINGLE ONE of these evap housings I've ever seen, was either crumbling to dust right there, or the dust had already fallen out and left a hole. The material they used for it just doesn't stand up.

Headers usually make it about 10 times worse, esp uncoated ones, since they usually come even closer and radiate even more heat.

I've often thought it would be wise to make a sheet metal heat shield of some sort for it, but never have got around to it.