I have a 99 blazer 4x4. The heater insnt working as it should, it doesnt blow very warm air. I have tried changing the t-stat and flushing the heater core, the core is free of obstructions. The other day while waiting in the parking lot for a friend, someone in another car parked beside me was smoking a cigarette and i could smell it just as if she was smoking in my truck (my windows were up and my heater was running) I was wondering where I could located the fresh air intake on this truck. PLEASE HELP!

I found some info on an air intake vacuum actuator, does anyone know where to locate this actuator?

 

The HVAC system draws outside air from the outside except when in recirc mode. The air is drawn in from the top of the cowl (above the firewall). Recirc mode is only available when the a/c is on.

Now your problem is likely a failing or failed air temperature actuator which is an electronic actuator that controls the blend door (or air temperature door). The actuator is located on top of the HVAC housing and accessible by dropping the glove box down and out of the way. The blend door controls air flow over the heater core.

Search should turn up quite a few threads discussing this.

This schematic shows the function of the air temperature actuator: