My 89 4.0 Renix blows hot air from the HVAC system regardless of where the temp lever is set at UNLESS I have the a/c on. I have checked the temp control linkage (it is working 100%). Before I start tearing things apart, I'd like to get other people's opinions of where to start. Should I start looking at mode doors, or should I start looking for vacuum leaks? Is there something I'm missing? Any suggestions would be appreciated....