* Wiring Issue: This is the most likely culprit. A break or short in the wire running specifically to that door's lock actuator is the prime suspect. Check the wiring harness near the sliding door itself, looking for damage, corrosion, or loose connections. The wiring is often pinched or chafed in the flexible sections of the harness as the door moves.
* Faulty Actuator: The electric motor (actuator) responsible for locking and unlocking that specific door might have failed. While less likely than a wiring problem, a malfunctioning actuator could explain the issue. You can test this (with some electrical knowledge and caution!) by directly applying power to the actuator. However, proceed carefully to avoid damage or injury.
* Bad Door Switch: While less probable if the remote unlocks work, there is a possibility of a problem with the switch itself inside that sliding door. This switch is the point of contact for the remote signal and could be internally damaged.
* Body Control Module (BCM) Problem: The BCM acts as the central brain coordinating everything electrical. A faulty BCM could selectively disable one door lock, although this is less common as a single lock would usually be low on its priority list for failure. This is more likely if other electrical components are also malfunctioning.
* Fuse (Less likely): While a blown fuse would usually disable multiple things, check your owner's manual to find the correct fuse for power door locks, and ensure it hasn't blown.
Troubleshooting Steps:
1. Check the Fuses: This is the easiest first step.
2. Inspect the Wiring: Carefully examine the wiring harness running to the passenger-side sliding door, both inside the door and where it connects to the vehicle's main harness. Look for breaks, fraying, corrosion, or anything out of the ordinary. Pay close attention to the areas where the wires bend as the door moves.
3. Test the Actuator (Advanced): This requires electrical knowledge. If you're comfortable with this, you can disconnect the actuator and test its functionality using a 12V power source. Always disconnect the negative battery terminal before doing any electrical work. If you're not comfortable, skip this step.
4. Inspect the Door Switch (Intermediate): If you can access the door switch assembly, you might be able to visually check for damage. This requires some disassembly of the door panel.
If you're not comfortable with automotive electrical work, it's best to take your Windstar to a qualified mechanic for diagnosis and repair. The cost of a repair is likely less than the cost of causing further damage while attempting DIY repairs.
