Did you splice your wires into the brake/signal lights? That's tha I would do. I'd also be sure to use water/weather proof connectoers and splices.

These contain epoxy that helps seal the connections from water. <img src="/forums/images/graemlins/kewl.gif" alt="" />

Alternately you could trace back from the light to the wiring harness inside and splice in there. You avoid needing the weatherproof connectors.