Ford Repair Questions? Ask a Mechanic for Answers ASAP
It should be 13+ volts, not 7 volts. Move your voltmeter back to the next spot and test again going back toward the battery or just start at the fuse if it's fused and work that way, but from where ever you start, when the voltage changes from 13 to 7, or from 7 to 13, in between those 2 spots will be where the problem lies and it could be a corroded wire, a shorted part, etc. Here are the wiring diagrams to help you determine where to test.
I would test again at the trailer running lamp relay pin 2 where the brown/white wire goes. If it's 13+ volts there, there is a S112 splice next to test at or near engine compartment fuse breakout.
If it's still 7 volts at that relay test the fuse #14. If the fuse is 13 volts then there is another splice s109 to test that goes to pin 5 of that relay where the yellow wire goes in.