Could be the harness, but also the reverse switch. If you have the 42re transmission (with 4.0), then you can easily replace the switch. On the 5-45 rfe (4.7) the switch is part of the solenoid block and the transmission pan needs to be dropped and the valve body removed.
JEEP Grand Cherokee 4.7 Lmtd 2004
HID 8000K in low and fog / Quad Squad member
KYB Gas-a-Just shocks / Pirelli Scorpion ATR 245/65/17 - Transgo reprogramming shiftkit.