I just found this wire flopping loose as I was starting to get my wiper reservoir out of the way to pull my blower motor. Looks like original and its been loose for quite some time. Its between the PDC and the outer passenger side. Its location suggests it could either connect to battery ground OR the PDC. It goes up into the bundle that goes over the wiper reservoir on the top of the firewall. Its large guage with an empty inline fuse socket. Would like to know what it is and if it could be affecting my other issues. 1995 XJ.

 

That is an aftermarket line. Assuming it was properly wired, because it's got the fuse it should connect to the Bat + (perhaps the B+ post in the PDC there). It was probably for an amp or lights or something the PO removed before he sold it to you. Unless you need the additional circuit, just leave be. As it sits, it won't be contributing to any electrical problems you may be experiencing.

 

+2. Not O.E. but aftermarket.