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The Ford Transit rear wire harness for the driver’s side tail light is connected to three ground wires located in the lower rear quarter panel. To get enough slack, remove the three grounding bolts, group the ground wires together with a bolt through the eyelets, and use a 6-gauge wire to extend the ground. This will relieve the tension and allow you to move the harness.
Depending on your van's year, see if you have a rear mounted light switch in the left door frame next to the wire harness, if so this needs to be un plugged (you will not be able to use this again unless you extend it as well) also check for metal clips holding the wires in place after unbolting and extending the grounds. Hope that helps!
Original question by @Teh8thWonder from https://www.youtube.com/watch?v=Rc9ht8rn5U8&lc=UgwE9d54tePETFzS6V94AaABAg