Half the lights stay on with switch off, sort of...

Ok, so yesterday I noticed that my instrument lights were on despite the headlight / marker light switches being off. After further investigation, I found that half of the marker lights were on, but were dimmer than usual. All of the lower corner lights on the tractor and trailer are on at half brightness, as well as the bottom side trailer marker / turn signal lights, and the tail lights and front corner marker / turn signals on the tractor. The upper lights are off, on cab and trailer, as well as the lower center marker lights on the trailer. Turn signals and brake lights work normally, and everything is fine when the marker lights / headlights are on.

Ideas? Anyone seen this before?

 

Durh, that would be nice to know, wouldn't it?

1998 Pete 377.

I also forgot to mention that there is no change with the trailer plug disconnected, and the lights come on with the switch off only when the ignition is turned to the ON position.