The best consumer book with electrical diagrams for 3-series BMWs is the Bentley manual. It is available on Amazon or from many on-line vendors of German car parts.
The most common problems with door locks are failing lock actuators and worn-out plastic snaps on the linkages. Neither of these should be affected by starting the car other than possibly the vibration.
With regard to your heater not working, we need to know how it behaves. Failing fan motor resistors are common on BMWs this old. If it is the heater control module, this link to my web page might be helpful.