Here is my first how-to, so here it goes.
I was having problems where my back-up (reverse) lights were not turning on on my 2004 M6S (6-cylinder) with manual transmission was shifted into reverse. While I am still having problems with this, I took pictures and got details on how to access and remove the switch.
Here are the tools you will need:
* 14mm combination wrench
* 21mm combination wrench
* #2 Phillips Screwdriver
* Continuity tester
* At least 2 differently-colored test leads with alligator clips
1. Open hood
2. Unclip 2-wire harness connected to driver-side of the airbox next to the fender.
3. Unclip the MAF sensor on the top of the intake tube (just forward of the battery)
4. Remove the accordion intake tube from the airbox by first loosening the clamp with the screwdriver, then gently removing the accordion from the tube.
5. Remove the air filter and airbox by firmly but carefully pulling up on the airbox. No screws hold it down, but pegs on the bottom of the airbox push through some rubber bushings which hold it in normally. This exposes the top of the transmission
6. Looking back towards the back and slightly to the passenger side, locate the backup switch. It is under the battery and just fore of the starter and the starter bracket as shown below:
7. The switch itself will take a 21mm combination wrench to remove, but you should remove the 14mm bolt just to the left of the switch just to make it easier. I noticed a bad smell after removing the bolt, (no, it wasn't me) and thought that it was probably from the loosened bolt.
8. Before removing the switch, go ahead and unclip the socket from the switch itself:
9. Unless you have a short 21mm socket that will fit, I found the combination wrench (the box side) worked well.
10. Keep the wrench handy as it takes quite a few spins to get it completely off of the transmission and my fingers weren't strong enough to unscrew it by hand only.
11. Inspect the switch itself. The travel of the plunger is only about a 1/4 of an inch. It should spring back automatically when pushed.
12. Also inspect the top of the switch to inspect for breakage or bent pins:
13. Test for continuity when the switch is pushed in by using the test leads (if necessary). If you get continuity, the switch is probably working. Screw the switch back in (finger tighten) and test for continuity when you shift into reverse. You should get continuity when shifted into reverse.
14. Test for resistance. If there is any resistance (anything more than 0 ohms), you may still have a faulty switch. If this is the case, you may have to replace the switch even if you read continuity when the switch is actuated.
15. Since the switch is connected by at least two small harnesses to the main harness, you could try jumping the pins on the side that goes into the car's wiring harness (and have someone step on the brake + clutch, turn the car on and shift into reverse) to see if the lights turn on. If it does, perhaps the problem is in one of the short harnesses mounted to the transmission.
16. Reassembly is as simple as following same steps from step 10 back to 1.