I'll bet the sockets are all corroded and crappy!!!! I'd buy a new kit.

But you can try this:

Connect the red wire to ground and each black wire, one at a time, to a hot lead and see if you can get the bulbs to come on. I'd assume the wire from the side tail bulb lights up the side light AND is connected to main tail light, too. The other two wires are stop / turn light and the ground wire for the entire assembly. My bet is that the red one is the ground (as odd as that sounds) but it may be the black coming from the main bulb is ground. Taking the lens off, you may be able to tell which wire is what.

BUT AGAIN... I'll bet the sockets are all corroded up!