How do i go about testing car stereo wires?

I recently re-wired my car stereo plug so it would fit into the back of my new head unit. Problem is when I went to plug it in it wouldn't power up. My old stereo powered up before I changed the wires. I may have connected the wires wrong. How exactly do I test to see which wires are in the right place and are working. Like the ground wire and power wire?How do i go about testing car stereo wires?
Well the best way is to take a 9v battery and a test light to the wires. The usual obvious power wires are Red %26amp; Yellow on most cars unless someone has changed them and the Black is common ground or you can use a frame ground which is just as good but make sure no paint is hindering the connection.
Take the test light and connect it to a frame ground and prob each wire and usually the memory is the constant hot and once you have that one located turn the car on accessory and probe the wires again for the turn on wire.
To figure out speaker connections, hook the 9v battery to each lead that is not a power wire and continue to test each one till you hear a crackle sound out of the speakers that will tell you which is which, as for which is + %26amp; - that requires hooking up the radio to do so since you normally can hear when a speaker is wired wrong the bass sounds all muddled and crappy.