In this video I will show you how to find your 12V constant, and your ign/acc wire to connect to the head unit's wiring harness. This method is for those who are installing a new head unit and the factory wiring harness has been cut out of the vehicle.
I highly recommend getting a circuit tester similar to the one in the video. It was pretty cheap (under $20) and saves a lot of time and guesswork. I recommend a multimeter as well, just so you can read the outputs and make sure they are correct. The only problem with a multimeter is that they can be confusing if you don't really know how to use them. The circuit tester is pretty self explanatory and really easy to use, connect to a good ground and touch the wire you want to test.
When using this to test for a constant, the light will come on when you touch the wire. For the constant, it should light up every time whether the key is in the on or off position. The ign/acc wire however, is a little different. The light on the tester will not light on this one until you turn the key to the on position. Once you find your constant, you can turn the key to the on position and check the rest of the wires. I would recommend connecting the constant as soon as you find it so you don't accidentally blow a fuse by grounding it out. That will also make it easier to find your ign/acc wire because it will be the only other one to light up.
This method is an easy way to make sure you don't get these wires backwards. If you connect them backwards, the radio will still work when the vehicle is on but it won't keep the settings once the vehicle is shut off.
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