In the wiring harness that goes to the back of the bike, one wire will carry switched current: the wire that feeds the tail (not brake) light — it's always powered when the ignition is on. If you can remove the tail light assembly, you may be able to see what color pattern that wire has — it's the wire that connects to the tip of the bulb.
If you have a voltmeter, you can find the wire this way. Connect one lead to ground, and the other (I'm guessing the leads have alligator clips on the ends) to a sewing pin. Turn on the ignition. Then using the pin, probe gently through the insulation on each wire. One wire will be carrying current, and will register on the meter. That's the wire you want.
If you don't have a voltmeter, make a test rig out of a small 12v lamp. Get a foot-long piece of wire and strip several inches of the insulation off both ends. Wrap one stripped end around the base (not the tip) of the lamp. Wrap the other stripped end of the wire around the sewing pin; this won't be a great electrical connection, but it should do. Again, turn on the ignition. Hold the tip (not the base) the bulb onto a bare metal part of the frame; your other fingers can help hold the wire that's wrapped around the lamp base. With your other hand, use the pin to probe into the wires of the harness. When you touch the live conductor, the lamp will light.