The only way I know of to be sure the correct insulation has been used is to apply a soldering iron to it. Otherwise, the two types of insulation have a very different texture and I can tell them apart. (Too bad whomever at Ford was tasked with QC'ing these harnesses couldn't). My suggestion would be to find an autoparts store that carries GPT wire and GXL wire and compare the two. Or check under the hood of any other car.
If the insulation melts upon contact, wrong insulation.
If the insulation does NOT melt, it is the right insulation.
I base this on some scientific testing done with a harness I pulled out of a junked '95 Contour and a harness that came out of an '89 Mustang.
Brian
