Instead of trying to tie into the tail lights use the trunk light socket by the heater/ac fan to control your relay. Easy to get to and it turns on with the tail lights.
I use it to drive my LED driving lights since they don't use much power.
Unless you want to mount the relay back by the engine. Then the tail lights are as good as any.
Be sure and fuse the line from the battery for safety.
[This message has been edited by Dodgerunner (edited 10-12-2013).]