Whenever I turn on my rear defrost in 2002 Mountaineer the interior lights come and then door ajar also up how can fix this?

The issue you're experiencing with your 2002 Ford Mountaineer—where the interior lights come on and the "door ajar" warning appears when you activate the rear defrost—points to a faulty ground connection or a short circuit somewhere in the electrical system. The rear defrost shares a common ground or power source with the interior lighting circuit and door ajar system. When the rear defrost activates, it's drawing a significant amount of current and this problem is causing a voltage drop or interference that's triggering these other systems.

Here's a breakdown of how to troubleshoot and potentially fix the problem:

1. Check the Rear Defrost Switch:

* Visual Inspection: Carefully examine the rear defrost switch itself. Look for any signs of damage, burning, or loose connections. A faulty switch can create erratic electrical behavior.

2. Inspect Wiring and Connections:

* Rear Defrost Element: Check the rear window defrost grid itself for any breaks or damage in the heating lines. A short in the grid can draw excessive current.

* Wiring Harness: Carefully inspect the wiring harness leading to the rear defrost switch and the rear window itself. Look for any frayed wires, corrosion, or broken connections. Pay close attention to the ground wires. A poor ground is the most likely culprit.

* Fuse: Check the fuse for the rear defrost. A blown fuse could indicate a short circuit somewhere in the system. Replacing it might temporarily fix the issue, but the underlying problem still needs addressing. If it blows immediately again, you have a serious short.

* Relay (if applicable): Some vehicles use a relay for the rear defrost. Locate this (consult your owner's manual) and check its condition.

3. Ground Connection:

* Multiple Ground Points: The rear defrost likely has its own ground point but may share ground connections with other systems like the interior lights. A corroded or loose ground connection anywhere in the circuit can cause the strange behavior you are seeing. Look for ground points near the rear hatch, in the rear body panel, or near the rear window. Clean them thoroughly with a wire brush and apply dielectric grease to prevent future corrosion.

4. Door Ajar Switch:

* While less likely, a problem with one of the door ajar switches (either the door switch itself, or the wiring) might be influencing the behavior. Check if any of the switches are faulty or show signs of damage. A dirty switch can occasionally lead to misreporting.

5. Professional Help:

If you're not comfortable working with automotive electrical systems, it's best to take your Mountaineer to a qualified auto electrician or mechanic. They have the tools and expertise to diagnose the problem accurately and perform the necessary repairs safely. Incorrect wiring can lead to electrical fires.

Important Safety Note: Always disconnect the negative battery terminal before working on any electrical components in your vehicle to prevent accidental short circuits or injury.

By systematically checking these areas, you should be able to pinpoint the source of the problem and get your rear defrost working correctly without the unwanted side effects. Remember safety first!