2004 GMC Safari Rear Air

If the rear gets a little col, but not cold it may be a tiny bit low on charge (134a). To properly determine this, a set of pressure gauges need to be used to se where exactly the system is at.When you change temp for the rear unit, does the blower sound change tone? If pressures are good it may be the blend door inside the rear plenum not moving all of the way or having an obstruction keeping it from closing off the heat. To verify this you need to remove the interior side panel and acess the HVAC plenum

Thanks for the info. However, it was charged with no change. I didn't understand about the heat as it is a stand alone with no separate temperature adjustment. Just works with the front A/C and uses a separate blower control to turn the rear air on.

So it doesnt have rear heat/air?

Correct, it does not have rear heat, just rear air.

Was it charged by a shop with gauges or a DIY kit? I would love to know the pressures. The rear evap core probably has an expansion block that isnt working, which would keep it from getting cold just in the rear unit

You were correct. The expansion tube in the rear unit was plugged. A new one was installed and it corrected the air problem. Thanks!

Youre welcome