Coolant flows through pipes under the body to the heater core in the rear unit. Probably the valve under your hood is stuck closed.

 

 

 

My '93 Astro only had A/C in the rear. I don't know about a '95 G. It shouldn't be too difficult to trace the lines to find out where a valve might be.

 

Bad how? If it leaked badly enough to stay cold, you would have a huge puddle inside your van. I can't imagine that your cooling system would have big enough debris to plug up a core. But I suppose you could hook up a garden hose to it and find out for sure.

Sorry, I don't know about your year. Just trace the lines forward from the rear heater. The front heater is on the passenger side of the underhood compartment. It should be close to that. Get your van good and hot, and use your touch. Once you find the valve, pull it out and test it. It will probably be vacuum actuated. Make sure it is getting good vacuum. Those lines tend to rot out.

 

On my '93 G2500 (which should be the same as your '95) the water control valve for the rear heater is directly behind the R headlight, under the coolant recovery tank.