Here are the installation instructions for the BX8869 Bulb & Socket tail Wiring Kit
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I had a Trailblazer with this kit installed. When you remove the tail lights you will notice a huge hole in the bottom of the light cavity. Just bring the wires down through the opening, up the frame, and terminate them in the appropriate location in the front of the vehicle.
The bulb drilling instructions including the drill size is detailed in the instructions. These holes are normally 1" in diameter. You will need a hole saw and arbor setup to do a good job. Finding the right spot in the lens isn't at all difficult. The location is basically on the side of the upper bulb reflector housing.
The biggest problem I had with the Trailblazer is that it always had a tendency to blow signal light bulbs. The TB uses a 3057/3157 which is plastic and glass base and for some reason is often burned out. I changed a lot of these bulbs so when you install the bulb kit leave enough wire (slack) for the Blue Ox socket so you can stretch the fixture out of its enclosure.
Good luck with your project and if you have any questions call Blue Ox direct they would love to help you. 888-425-5382 or 402-385-3051
Your expected response time will be as follows:
Email: Response within 3 hours
Fax: Same day
Letter: Next day