I've got my headliner out for a reupholstering, and I noticed this unused wiring harness above the passenger side C-pillar. Did some searching but couldn't find any info on it.

Anyone know what it's for? Rear A/C maybe?

https://imgur.com/a/oqvwBcV

 

Never had a truck with rear air, but I have had trucks with an alarm sensor in that location.

 

Actually, now that you mention it...when I was following one of the various step-by-step headliner guides out there, there was something about alarm boxes that needed to be pulled out around the B-pillars on both sides. But I never found anything like that.

This is further back, and only on one side. My D2 has an alarm. Not sure if there were more advanced options in 2001 than what I have.

 

I have both rear air and the alarm sensors. The alarm only has two wires but there's a controller unit in the centre of the roof above the passenger seats for the rear air controller. My guess is it's the latter.