Did you try this test? This is from an earlier post....
3. Your neutral safety switch (by the transaxle) is either dirty, needs to be adjusted, or just really worn out. If it's dirty, it can be cleaned. If it's worn out, you have to get it replaced and it's like $450 at the dealership! Try this test...tell someone to stand behind your car and shift through Park, Reverse, and Drive really slowly, of course with your foot on the brake. If the reverse lights come on for a slightest moment, most likely it just needs to be adjusted. It's a matter of loosening a bolt or two, moving the switch, and then retightening it.
you should be testing for ground. i think its a one-wire harness on the transmission near all those harness are at, right infront of the radiator. you might need to remove the cruise control to reach to it.
Remember to do the test first before pulling apart that switch. If you really can't find the switch, follow the cable that is connected to your shifter that goes into the switch and transaxle. Basically, tell someone to shift through the gears, you see some cable move in the engine bay...just follow that down. It's connected to the manual shift lever which goes into the neutral safety switch. It's a simple part, but it costs $450 Cdn to replace.