I've been looking at this problem for months and still no luck.

Head unit is a pioneer.

It has 4 wires on the car loom

Red - Guessing 12v
Black - Guessing ground
wire 1 & 2 - nothing on multi meter

Can someone point me in the direction of a thread which gives a definitive answer on how to fix this? Ive search but without pictures they all seem very confusing.

The other two wires should be ignition live and illumination, they usually make up the four power wires for an aftermarket unit. The head unit will need both the ignition and permanent live to work (turns on and off with ignition, retains memory with permanent) the illumination isn't important, it just dims the display when you put your lights on. Double check which is which with the meter, one should go live with ignition (usually yellow) and one with ignition and lights.

Basically you need to connect a wire from an ignition live like cig lighter or the blue wire on ignition barrel to the ignition live on the headunit, connect up as normal except for the wire your adding the headunit normally a yellow wire