As some of you know, I've got Russ's old Ami 6. Originally it had an M4 engine in it, but now it's got a later engine I'd like to connect up the oil pressure switch. How do I achieve this? The wiring from the switch is all there, I just need some advice on how to connect it all up to a little light on the dash. Cheers ...

The oil pressure switch on the engine earths the circuit.

So you want to take a live from the ignition switch's "run" position, run it through the bulb, and off to the switch. That simple.