* Faulty Rear Wiper Motor: The most straightforward reason is a failing rear wiper motor. Intermittent operation suggests a problem with the motor's internal components (brushes, windings) that's exacerbated by heat or wear. The motor might also be drawing excessive current when failing, leading to the dead battery.
* Bad Wiper Switch: A faulty rear wiper switch can send erratic signals to the motor, causing the intermittent wiping and the failure to return to the parked position. This wouldn't directly cause a dead battery, but a short circuit within the switch *could*.
* Wiring Issues: Corrosion, broken wires, or loose connections in the wiring harness leading to the rear wiper motor are common culprits. These can cause intermittent operation and excessive current draw (dead battery). Look for chafing wires near the tailgate area, a common point of wear and tear.
* Grounding Problems: A poor ground connection somewhere in the rear wiper circuit can cause erratic behavior and excessive current draw leading to a dead battery.
* Rear Wiper Fuse: Although less likely given the intermittent nature of the problem, a failing fuse could also be the cause. Check the fuse box for a blown fuse related to the rear wiper.
* Body Control Module (BCM) Issue: The BCM controls many electrical functions in your Expedition. A malfunctioning BCM could send incorrect signals to the wiper motor, potentially causing both the erratic operation and excessive current draw. This is less common but possible.
* Power Issues (Alternator/Battery): While you mention a dead battery, ensure your alternator is charging properly. A weak alternator will not adequately recharge the battery, particularly if the wiper motor is drawing excess current due to one of the above problems.
Troubleshooting Steps:
1. Check the Fuse: Locate the rear wiper fuse in your fuse box and inspect it.
2. Inspect the Wiring: Carefully examine the wiring harness leading to the rear wiper motor for any signs of damage, corrosion, or loose connections. Pay close attention to the area near the tailgate.
3. Test the Wiper Motor (Advanced): This requires disconnecting the motor and testing its operation with a direct power source (carefully!). This isolates whether the problem is with the motor or the circuit.
4. Inspect the Wiper Switch: If you're comfortable, you can test the switch's continuity with a multimeter.
5. Battery and Alternator Test: Have your battery and alternator tested at an auto parts store. A weak battery or alternator will exacerbate the problem.
Recommendation:
Given the interconnected symptoms, starting with a thorough inspection of the wiring and grounds is a good first step. If you're not comfortable working with car electrics, take your Expedition to a qualified mechanic for diagnosis and repair. It's safer and will likely save time and frustration.
