the brake light power goes to the lights via the turn switch, when you select a direction the switch activates that light or interrupts it if the brakes are being applied - if both rear [brake] lights come on with the signal activated, you have something mis-wired - brake and turn are not 2 separate circuits - here's a schematic for an add on unit, and here's the factory 55.2 diagram

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics