g'day im new to this forum but came here because haven't had any help from other audio forums..i've wired up a couple of headunits in my time but thats been thanks to a colour coding chart. I've been asked to do a mates car and just wondering how would i diagnose what wire colour is what? e.g telling what constant 12v, accessories, ground and the speaker wires regardless of colour and what tools would i use to check what is what and how? cheers

google it?

what sorta car? and year and model

Pickup a Multimeter from an electronics shop, doesn't have to be anything fancy, but they are a great tool to have

You can probe the plug till you either find voltages with turning ignition positions (power sources) and headlight illumination or resistance on speakers wires.

Finding what speaker is which might require tapping the wires on a low power battery to hear popping once you are certain you've found speakers

$10 multimeter will do the job... buy one, good investment to have. You can then find accessories power by using the probes with the ignition on, and constant power with the ignition off.

As said, get a multimeter first up or in a pinch one of those test probe lights will do.

With everything switched off there will be one wire that shows 12V+, thats your constant power. Next turn the keys to acc on the ignition and there should be another wire that shows 12V+, this is the accessory (switched) power wire. If you hook these up backwards the HU will work but you'll lose all the memory settings when you turn the car off.
With the acc still on, test all the other wires with and without the headlights switched on, this is your illumination wire.
Ground is most often black but its always a good idea just to run a fresh ground to bare metal attached to the chassis of the car.

Speaker wires are sometimes paired in colours, sometimes they're are twisted together which makes it easy. Sometimes they are totally random which makes it a pain.

Other wires can be for powered antennas, car kits etc.

Google is your friend for model specific info.

i use the above method KGB has stated and use a D size battery with a short piece of wire soldered to each end to find the speaker wires, when you touch both the battery wires to both wires on one speaker it will make a cracking/popping noise. if the speaker moves out that means the polarity(positive and negative) is correct on the battery if it pops in it means it is back to front. also another way to tell the polarity is that normally the darker wire is the negative wire of the speaker if the wiring is still factory. only a rule of thumb though.

Cheers, Grant