To properly wire up your CHMSL on your cap, look under the rear of the truck on the drivers side just to the left of the spare tire. There is a white wire with a tan tracer that is taped up to the outside of the harness. This is the tap for the L50 CHMSL circuit (page 8W-51-6 in the 2006 service manual) straight off the brake lamp switch.
This tap is the proper place to wire in the CHMSL, or if you have one of the trailer hitch covers with the LED's.