I'm finally getting around to hooking up the correct antenna adapter for my head unit, but I'm stuck. I need a 12v power supply to splice the adapter to, which wire can I tap into?

Thanks

 

Use the remote wire from the aftermarket H/U. I believe you need a 5v not a 12v wire, which makes the remote wire perfect. If you require 12v you could always splice into the H/U's power wire, or you could run a wire to the fuse box and attach it to something like "ACC" or a Cigerette Lighter Fuse, those will both be 12v.