If you dont mind me asking, how are you metering these wires out? 1 test probe on the brake wire the other where? Make sure it is a good source to ground. (A quick continuity test will prove that too). When in doubt, go straight to the battery -.
If you metering it properly (not trying to insult you, some people just dont know) then check the fuses and connections to the fuses for a bad connection. Test for voltage there. If that proves ok then we have a decent problem.
As of that point, Pick an end and work the other way. So if you do not have anything at the tail light, work your way back to the quick disconnects right there. No power there? Back to the control switch. (Brake lever/blinker relay).
Pull out the wiring diagram to see the exact order of the wiring. If you lived close to boston I'd be glad to help