troubleshooting dual zone heater on '93 Regal

I just bought a '93 Regal and the passenger zone heater is not working correctly. The temperature control does not seem to work. I put it on high vent/heater/dual and it only blows cold air. However, if I switch to Max AC and leave the heater up high, it will then blow hot air.

I know in many models the mix is controlled manually. I was wondering if the Regal uses some sort of electronic switch or what the deal is and if this is a quick fix or if I need to bring this to a dealer.

Many thanks.