I’ve dug in to it a little more and think that this is a plug for the “thermostat monitoring sensor.” Seems to be in the right place and the colors match up from the wiring diagram. The RAVE says the sensor is fitted in the radiator by the lower hose, so check if there is an open connector there. I’ve attached an image from the electrical manual, the circuit diagram, and the RAVE if that helps.

 

OP says in the first post that the coolant temperate monitor sensor is connected.

 

Is the D2 equipped with SAI? They have a second temp sensor located on the radiator. Someone might have replaced the radiator with a non SAI unit and they just left the wire hanging.

 

Crap, how did I miss that? I really thought I was able to crack this one. Unusual that there would be a plug with the same combination of wire colors in the same area.

 

Maybe he's got the wrong temp sensor cable plugged into the one receptacle?

 

There is the ambient temp sensor, horn, and SAI temp sensor all in very close proximity on the passenger side.

 

Thanks for all the responses. Both trucks are SAI, and the radiator in the 03 had a plug in the radiator where the sensor goes with the connector left dangling. The PO of the 03 told me he would occasionally get a code for temp out of range or something and couldn't figure out why. I presume this was why. When put the 03 back together I used the radiator and engine harness from the 04 and reconnected the same connector to the sensor in the bottom of the radiator. The temp sensor in the radiator is this style:



and the connector in question (In the initial post) is identical in style to the connector for the horn. This is definitely part of the chassis harness and not the DME. I'm wondering if greisinb is correct and this mystery connector is for something not on my truck. Aux lighting is a good guess based on the type of connector. When the rain stops I'll put a meter on it and see if I get voltage.