hello Steve,

i face the same problem,but only from time to time.

cold wind only,airco engaged even whenn put at max heat.cured sometimes by sitching engine on and off.

start with the easypart, the vacuum controlled valve at the rear of the cyl.head.( the little round golden bulb.

it should CLOSE whenn asked for heat,and open for cold air.

this part gets spoiled quite often,

you could replace by a plastic valve,doing the same,costs a fraction and lasts longer.

it did not solve my problem yet,will keep you informed on the research.