It could very well be the heater control valve that is malfunctioning.
This is a really common problem in the Vanagon.
I chose to install a manual ball shut off valve behind the heater face
plate.
It's a hand ripping affair changing that buried valve up above the spare
tire carrier, and in my opinion a pain to change.
I understand there are some miracle worker's who possess the ability to
swap this cheap valving system in no time at all, but I found that it
always isn't the valve, it's the cable retainer that allows the cable to
slip and deem the valve useless, hence the decision to install a manual
valve inside the cab.
Problem solved.
However, if your getting nothing but cold air out of the front heater
consider a plugged heater core which is a good possibility with age.

E-mail message

Subject: Cold air always

Must be one of the "odd" things about these vans. Our old 85 GL had the
opposite problem with nothing but really hot air coming out - especially
in the summer. We ended up having to have the vent shut off somehow
during the summer - I think our mechanic did it. Never did solve the
problem.

Sharon & Bob Mendonca