rear heat issue

My 1994 lumina apv blows cold air out of the rear heater ducts when the rear heat is in the on position. I have good heat in the front of the van, but have no idea why there is not heat coming out of the rear. Any help would be appreciated and thaks in advance.

FIrst to get heat from the rear, your selector up front must be in the HEAT or DEF position.
Lately, there was a thread for similar problem: the rear core was proven clogged.
Finally, there is a vacuum driven valve near the firewall (you can barely see or access it) and tbhis valve controls the coolant flow to the rear core; it may have come apart (like mine, '93), the vacuum drive separated from the valve itseelf.

Thank for the advise. I will check the items you mentioned

.. rear air is directed up to roof ducts in VENT or MIX (floor/vent) mode , and will ALWAYS blow cold only, whatever temp setting in front
in HEAT or DEF/FLOOR, rear air goes to floor and will (should) blow warm whatever the front temp setting.
..all of this, of course, if all is normal. If you have air only to top vents, then coolant-to-rear valve is not activated either.