I did it. I needed three GM parts for the conversion, the Homelink unit (22850908), button pad (22739808) and the central trim plate with openings for sunroof and garage door buttons. The problem is that you cannot buy just the trim plate. I had to get the whole overhead console assembly and transfer what I needed to my console.
I found one on ebay that had what I needed. It was an NOS unit for an XTS but will actually fit into an ATS. I mention this because it has features that mine doesn?t. It has a switch in the rear passenger side for a rear sunshade. It also has LED map lights while my originals are incandescent. LED lights have circuit boards where light bulbs would be and lens texture and color are different. I tried both lights in my car and decided to stay with the originals. The LED?s blue shade made my caramel interior look grey. The incandescent bulbs have blue tinted glass but the color was a better match for my interior. Maybe I?ll post these LEDs over in the classifieds since the electrical connector and fasteners are identical to the ATS.
My wiring harness doesn?t have a connector for the Homelink so I had to splice one in, which wasn?t a big deal. I bought male and female plug kits at Radioshack for this and used three wires.
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I spliced into a map light wire loom. With the console removed you can see that the black wires in the connectors are ground. All of the yellow wires are for symbol illumination and are dimmed with the dash lights. I used that for the Homelink button illumination. I connected 12V positive to the white/brown wire in the map light. It gets power as soon as the door opens and power goes off after 10 minutes or so to conserve the battery. I soldered wires from the other connector to the Homelink unit. The ground wire goes to the back terminal, green illumination wire to the center and red 12V power to the front. I wrapped up wires and exposed conductors with electrical tape before installing the Homelink to the console.
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I reinstalled the console, programmed the garage door opener and then had an adult beverage to celebrate.