The Problem is corrosion in Junction Block Wiring Connectors
It took me 12 days to fix this after a minor thunderstorm with extended heavy wind driven rain. On the passenger side of the Jeep, under the passenger kick panel, under and to right of the glove box near the floor pull the plastic panel off held on by plastic screw. there are 2 wire junctions in there with a white plastic cover - use a 10 mm socket to unscrew the bolt and pull them apart and see if they are corroded and have un-evaporated water inside them. there are 2 wire harness junctions. the upper harness was water damaged and corroded (green corrosion) on my 2004 Grand Cherokee WJ Laredo. the water runs down the wire bundle into the male part of the connector, and flows through terminals to the female. the solution was to use electrical contact spray and clean well,dry and reassemble. you must clean behind as well, remove the female part of the connector as well from the black plastic holder and clean behind that as well.
My symptoms were:
1. The Jeep will start briefly for three seconds and then shuts down. On repeated attempts the Jeep turns over strong but won't start and there is a smell in engine compartment after trying to start it. After disconnecting the battery or opening the rear passenger door the car will start briefly again and the interior lights will work on opening the door again.
2. Power windows not working, interior lights not working on door opening, locks and flasher lights not working from wireless remote (however a clicking is heard in the back when pushing button), overhead console not working - just dashed lines, on Instrument cluster some gauges (battery, fuel, etc) not working and various warning lights (such as sentry key light) eventually come on.
I tried the following to no avail and discovered the following:
1. Disconnecting the passenger door module, main computer, BCM, cam position sensor.
2. Disconnecting the battery overnight.
3. Black PCM computer connector: A22 (direct battery) reads same voltage as battery instead of proper 0.5-2.5V. A2 (ignition) reads same voltage as battery. A31 (ground) ohms out at 0.02
4. Grey computer connector: C30 (communication line) reads 0.01V
5. Cam position sensor reads 5.35V on both pins
6 I tried disconnecting the passenger door module.
7. 16 way diagnostic connector. J1850 Purple with yellow tracer at pin 2 is communication on the PCI bus and reads battery voltage instead of proper 0.5-2.5V