E30 Zone

I have a few nasties on my M42, when I start it the heater and all gauges come on, then after 30 seconds they all shut down leaving no speedo, fuel gauge, rev counter or heater motor.
What could this be ?
Ps upon starting the car fuel gauge is always on half but it doesn't have half tank in .

Posted in the wrong forum - a M42 is an engine, and doesn't have a heater or gauges. These are part of the car, so moved here where it has more chance of being answered.

Second question first - someone has fitted a fuel gauge from a car with a 63 litre fuel tank into a car with a 55 litre tank.

Does anything on the car work, apart from the engine, when the first fault manifests itself? Lights? Wipers? Door mirrors? Sunroof/windows?