What would be making a constant ringing like when the key is left in ignition and door open as you drive your 99 Chevy Silverado?

That constant ringing sound in your 1999 Chevy Silverado, similar to the key-in-ignition warning, even when the key isn't in the ignition, is likely a faulty body control module (BCM) or a problem with its wiring harness.

Here's why:

* BCM's Role: The BCM is the brain of your truck's electrical system, controlling many functions, including the warning chimes. A malfunctioning BCM can cause it to issue incorrect signals, resulting in a constant ringing even when there's no actual fault condition (like the key being left in the ignition).

* Wiring Issues: A short circuit, damaged wire, or corroded connection in the wiring harness related to the BCM can also cause this false warning. The wiring related to the door switches and the BCM is especially suspect. Sometimes, just a loose connection can be the culprit.

* Other (Less Likely) Possibilities: While less probable, a faulty door ajar switch or a problem within the instrument cluster itself could *potentially* contribute, but they're less likely to produce this specific, persistent ringing sound.

Troubleshooting Steps (in order of easiest to most difficult):

1. Check all doors and the tailgate: Make absolutely sure all are securely closed. Sometimes, a slightly ajar door can trigger a persistent alarm.

2. Battery Disconnect: Disconnect the negative terminal of your battery for a few minutes. This will reset the BCM. Reconnect and see if the ringing stops.

3. Inspect Wiring (if comfortable): Carefully examine the wiring harness in the area of the driver's side door (where the door switch is located), paying close attention to any signs of damage, corrosion, or loose connections. This is a bit advanced unless you're comfortable with car electrical systems.

4. Professional Diagnosis: If the problem persists, take your Silverado to a qualified mechanic or auto electrician. They have the tools to properly diagnose BCM issues and wiring problems. They might use a scan tool to check for diagnostic trouble codes (DTCs) that can pinpoint the fault. The BCM may need to be replaced, reprogrammed, or have its connections repaired.

Remember safety first when working with car electrical systems. If you're not comfortable with electrical work, it's best to leave the diagnostics and repair to a professional.