This is a real interesting subject. I just bought a brand new 2015 transit connect 1.6 TDCi (this is the biggest engine allowed in this car here in South Africa), and I would also like to know where the rear speaker harness is. I opened the one panel on the right side where the fuse box is located, and saw an unused harness, but not entirely sure if it's the wires I'm looking for. I'm too afraid to hook them up, and maybe blow something costly in the process. So, maybe your harness is on the same side. You're welcome to try it out and let me know if it works, if you're willing to take the chance. Me personally, I don't have the balls I guess ;-)