Rear Air/Heat? (Tahoe)

[92chev]

My 1999 Tahoe OBS will only blow cold air out of the vents in the rear even with the heater on.

When I have the control set to split air between feet and vents It'll blow warm air at the feet in the rear, but cold out of the vents in the rear at the same time.

Whats up with this? I dont really know how the rear air/heat works on this thing, so maybe someone could clue me in. All the fuses appear fine. Whats wrong with it?

[JWolfe75]

one of the lines coming out of the front heater core goes to a little plastic valve on the passenger side of the engine bay. Its vacuum operated, make sure its hooked up and that it opens when you turn it to heat.