All my exterior lights work properly except for the RH front turn signal. Not only does this signal not work, but when it's activated, the front headlights come on (?). Measuring voltages at the two front turn signal sockets gives 2v when the inoperable side is supposed to be on, and zero volts when it is supposed to be off. The working LH turn signal socket gives 12v when it is supposed to be on, and... 4v when it is supposed to be off (!). This 4v is constantly present, regardless of whether or not the turn signal switch is activated or if the key is in the ignition.

I've just removed the steering wheel today to check the wiring on the back side of the turn signal stalk. All wiring appears intact and solid, no broken or frayed wires. Does this sound like it could be a bad body control module?

As a possible related issue, I'm also having a battery drain problem. If left connected with the car not being used (as I've had to do while doing body and paint work) the battery will drain completely dead in two weeks (~10v resting voltage). I've watched the voltage on the connected battery on a daily basis now, some days there is no change, but sometimes in a 48 hour period battery voltage will drop from a fully charged 12.7v down to 12.3v. I'm starting to think that a flaky BCM is draining the battery. Any input would be appreciated.

 

Could be something as simple as a short in the wiring.

There are a lot of wires in that bundle coming out of
the BCM.

 

It could be in the turn signal stalk itself. Some people have had problems with it. Try a search, just a thought.

 

Just checked, there is also 4v constantly present (with key out of ignition) at both rear turn signals and both rear taillights. The two brake lights have a normal zero volts.

 

4 volts between which two points? And are you saying that the lamps with 4 volts on them are not glowing dimly?

 

4 volts at the socket connection points (where the bulb plugs in). No, the incandescent bulbs don't glow at all with the 4 volts applied. When I get home from work, I'll see if this 4 volts is still present with the bulbs plugged in and isn't being pulled down by the load of the attached bulb. This will be possible to do on the rear lights because of exposed measuring points on the back side of the rear bulb assemblies. Also, I'll try to measure current flowing through the attached bulb.

 

Do you have a wiring diagram?

I think this version is pretty close for lighting.

 

This turned out to be a dead simple fix. The 4-way connector (between the washer fluid reservoir and the bumper cover) was dirty. After being cleaned, the turn signal now works. The front undertray splash shield was missing when I bought the car, and was probably driven for some time that way. As this connector is exposed to spray from the front wheel if the shield isn't there, it was packed with silt and the male prongs were quite corroded.

Needless to say, I've just ordered a replacement undertray. I'm also going to check and clean all of the other connectors that might have gotten the same treatment.