1992 Nissan 300ZX Blowing Hot Air

I have the same exact problem with an 87 300zx and dont know what to do either.

Update: I pulled the bower box from behind the glove box. Upon inspection there was vacuum actuator that the diaphram had deteriorated out. Its complicated expensive piece. All it does is open/close the inside/outside air door. In its relaxed position it draws outside air. I disconnected the actuator and re-attatched the spring to always be drawing inside air instead. There are dozens of variables that can cause hot air instead of a/c air.
The heater core valve on the engine side could be
broken/stuck. The vacuum supply to the valve could be failed. The control panel that tells the valve to close could be failed/failing. Since its a control panel
to regulate by the degree it needs the valve to operate in increments, if its not doing its job too much hot gets mixed in with the cold. The seals from the blower box to the a/c box are gone by now and the blower cant "force" air through the evaporator and into the ducts. Cant tell you what to do, too many variables.