R31 Stereo Installation. Help!

[GXE23]

HELP!

I've spent the past week trying to nut this shit out, but I'm getting no where. To skip over a lot of details and cut RIGHT to the chase.

I am trying to complete my stereo install on my 1989 R31 GXE.

I have re-run speaker wiring to doors/parcel shelf etc. I cannot for the LIFE of me (Without fear frying my head-unit) figure out which is 12V constant. And Which is 12v Accessories/Ignition.

After looking on multiple pages/sites/forums, I now know that BLUE/BLACK are the Accessories/Ignition & WHITE/GREEN is 12V Constant.

However, I have 2 white/green and 2 black/blue wires:

Also I was told investing in a Voltage Tester would help. I have uploaded a picture of the one I have which works on 240V accessories but will not register ANY current on any of the wires on my car. Is this perhaps due to the type of Tester I have, as it doesn't have a "ground lead"? So ultimately, buying the tester has gotten me no further along with something that should be so simple.

If anyone at ALL can help me with nutting out these last 2 wires, it'd mean the world.

[pkiss01]

can’t remember from when i did mine, best way to work it out is grab a circuit tester( cheap screw driver looking thing with a light bulb in it and a lead coming out of the top) and probe the wires to find the constant and the acc/ignition. when the light bulb turns on there is power running through the wire the metal case that the headunit bolts up to is a good ground.