Do you smell something sweet like coolant inside your Vanagon? Notice any new never-ending wet-spots on your carpet? Your Vanagon might have a a rear heater core leak.
Replacing a Vanagon rear heater core is a simple do it yourself fix. You will need to purchase a new rear seat heater core, core control valve, and control valve o-ring. The cost should be no more than USD $150 with shipping.
In order to remove the old Vanagon rear heater core, you must first get past the plastic housing and protruding air vent. The housing must be cut out by making two small cuts on both sides of the rear heater vent. By cutting out the horizontal lower piece of the heater core housing cover, it will allow you to lift out the housing and expose the heater core.
The rest of the job is straightforward. Remove the old core, control valves, and o-ring. There will be some coolant that spills on to the floor, and if you have recently driven your Vanagon, it will be HOT. Before inserting the new heater core, take some time to clean out the rear heater assembly, air vents etc, there may be years of accumulated dirt back there.