take out the back seat. part alone is 150. its covered 4yr 60k miles. if out... good luck
I just did this myself on my 2003 78k g35 Bose sedan (no spoiler) for only $2 and about three hours:
1. remove the ceiling panel in the trunk and unplug the light. it has only two wires. after unplugging you can depress the brake pedal and see with a multimeter if there is voltage in the female end coming from the car. I only did this to verify that the light was the bad part.
2. remove back seat. the bottom part has two pull tabs. then get a deep-socket 14mm and ratchet with extension to remove seat backs. There are two on each side. it will also help to remove the three seat belt anchors with the same socket. keep track of parts and relative torque on bolts so you can put everything back safely. lift up on seat-backs to remove.
3. lower arm rest and pull up on velcro lip to expose two more bolts to remove. then work out center plastic piece with screwdriver and rocking side to side. armrest should come out.
4. on either side of armrest frame, remove two plastic fasteners on each side. be gentle when pulling back the insulation/foam back mat since it tears easily.
5. open the little door to the trunk and remove the door seal. then lift the whole armrest frame out.
6. remove six exposed plastic fasteners with screwdriver. also remove the 3 babychair top anchor plastic covers.
7. jimmy up and toward you the back dash panel. there are some other tabs and buttons holding this in so be careful and patient. push down on the far sides of the panel too. hard to describe all the pieces holding this in.
8. I found the easiest thing to do here was to turn it upsidedown in my lap and the third brake light should be right there. remove the two screws with a Phillips. be gentle since I broke one of the female ends and had to crazy glue back into place.
9. third brake light should now come out. grab a multimeter and troubleshoot. for me, it was the black 1n4007 diode that wasn't working. I verified this by bypassing the diode with a clamp (or thin wire) and plugging the unit back into the trunk. stepped on the brake and it all six LEDs illuminated.
10. got an assortment of diodes from Radio Shack, including a 1n4007. without even removing the old diode I soldered the new diode in paralled (white line on diodes should be in the same direction. http://imgs.tootoo.com/img01/products/119/3564119356.JPG
11. plug in and test again with new diode in place.
12. put everything back together. hardest part is that back dash panel. there are various foam pieces that will help keep vibrations down fromt he rear subwoofer. also be safe with all seatbelt parts. I also took the opportunity to clean and condition all the leather parts before putting them back in.
While user User:Robanddan9recommends the above steps to remove the 3rd light, I would do the steps below which will take about 20 minutes:
1. lower the trunk ceiling panel by removing the plastic tabs with a flat head screw driver.
2. remove the insulation around the light
3. remove the screw on the driver's side
4. unplug the light - to do this, there's a tab on the side that you have to slide down
5. drill a hole to access the screw that is impossible to access opposite the driver's side screw. Its MUCH faster to do this than take out the seat, etc. I used a drill bit that starts out small and then creates about a 1/2" hole. Make sure you get the right bit that will drill thru metal and the plastic. If you've removed the insulation you will be able to physically see the screw from the trunk which makes it easier for you to estimate where to drill the hole. I also removed the speaker cover to provide more light and access to pull the LED light out.
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