Last summer, I put a new Aux fan in my car and had my mechanic recharge the AC. Everything checked out. Pressure good, no leaks. But, the mechanic told me the air coming though the vents never got as cold as it should. Sure enough, it only gets slightly cool when running below freeway speeds (about 50mph). If I take an extended freeway trip, it's gets much colder, more like how it did when I first bought the car.

He suggested the heater core area might be clogged and wanted to rip into the dash to find the problem. I said "no." Another mechanic friend suggested that the radiator might be really dirty/clogged, but wouldn't the engine be overheating if that were the case? Any other suggestions as to what I should look for. Or how best to tell if that mysterious area between the blower motor, the pollen filter, and the heater core is the culprit?

thanks in advance!