Building a Vintage Car Microprocessor Controlled Climate Control System
If I understand correctly, you want three knobs and some switches to control a fan, and to direct the inlet air source.  That's easy enough for a microcontroller if the interfaces exist.
You'll need a motor-control interface to control the fan, and if I understand, some mechatronics will be required to control the inlet-air-source.  It would be nice to understand how this can be motorized or solenoid driven.
MCUs typically run at 3.3v or 5v, so you'll need to regulate that down from 12v and filter it appropriately.  The power-supply for the drive electronics for (fan and inlet control motor) will depend on their specifics, but will likely include digital (PWM) outputs driving an h-bridge as is common in motor control applications.
If you want to proceed with it, I suggest you document the system clearly, focusing on its inputs (knobs and switches) and its outputs (the fan and air inlet control motor).  Also describe all the expected behaviors.  Such documentation will help you greatly when working with others.
To program and test this you would likely want to build a desktop development rig with duplicate controls, fan, air-intake control system and drive electronics.