The reverse lights are a known problem on this generation of Jeep Grand Cherokee. 99-2003. The factory switch that activates the reverse lights is part of the valve body inside the transmission. When it fails typically the only problem is the lights not working, so I was not about to pay the $400+ for the valve body and open up the tranny.
Here is what I did to get the lights working and pass my state's inspection.
Step 1: Locate the Trailer Wiring Harness
It is located in the drivers side rear most cargo panel. This is where I got power from and also where I tied into the back up light circuit.
Step 2: Use a 12 Volt Relay to Make Connections
I used a Bosch 5 pin 12 volt relay
this wire bundle is a great place to tap into becasue it as all that you need to compete the job.
There is a constant 12v, Tap into that source and connect Pin 87 and 86 (86 would normally be switch input, we are going to use ground as the trigger) I used 3m Scotchloc to attach to factory wires.
Tap in to reverse line and connect to Pin 30
Now connect a wire (black is typical for automotive ground) to pin 85 This is what will run to the new switch.
Use a self tapping screw to attache the relay.
Step 3: Run Wire to Center Console
Route the new black wire installed up to the center console it under carpet or along the doors under plastic moulding. Sorry I did not take a pic of this step
Step 4: Remove Center Console
WJjeeps.com is a great source for info on how to do this step.
Step 5: Make Bracket for Micro Switch
I used some scrap 1" flat bar (1/8" Thick) bent in to a stair. Then two self taping screws to hold it the body.
Step 6: Attach Mirco Switch
I used two tack nails to go though the switch body and into two holes I drilled in to the bracket. The nails act as pins and then I hot glued it together.
Step 7: Make Trigger for Switch
I used Plastic epoxy putty for this, It was used is to fill the gap between the micro switch and the shiftier, also it provides a small enough contact point (shaped like a triangle) so the switch is only activated when in reverse.
Step 8: Connect Wires to Switch
I soldered the wire on to the contacts of the switch, one is the wire we just ran from the relay and the other goes to ground. There is a bolt in the bracket for the parking brake handle that I used to attach a ring terminal to.
Step 9: Test Switch
Run the shiftier through the gears. Make sure that the switch does not bind the shiftier any any point. Then confirm that the switch is only activating when the shiftier is in the reverse position. You should be able to make small adjustments buy bending the bracket carefully.
Step 10: You Are DONE!
After testing put back any trim removed and reinstall the center console.
Grab a cold one and admire your work!