I am starting to wire my car, and I don't won't to put a battery in just yet. I am doing a complete re-wire and would like to be able to test as I go. Any ideas without a battery? I have a low amp trickle charger, could I use that?
Use a continuity test at first. If you have continuity, you'll know voltage will pass. That said, you can have continuity and still not be able to pass the required voltage, so this is just a test. Once you get all your wires in, you'll be able to load test with the actual battery.
If you want, you can wire in a tiny 9v battery and use that to test your bulbs and such. They won't be bright, but you'll see a glow at least. Obviously, it won't be able to power the wiper motor or headlamps, but the other low amp draw circuits will work. And the amp output of that 9v is so low, that you won't start any fires if you get something really wrong. But in reality, you don't really need the 9v setup since checking continuity will get you on the path to start testing with the real battery.
Do you have a riding lawn mower battery you could borrow? Another option would be hooking jumper cables to your daily driver to the battery cables in the Mustang. I have also done this with a battery charger hooked up to the cables with no battery. I know that wasn't the greatest idea, but it was only for a few seconds.
Do you have any older portable electronics? The wall plug for older stuff used to be 12v for everything, you plug that in and connect to the battery terminals. Not alot of amps, but enough to check circuits. RC car batteries are 12v too, you could use one of those chargers.