I have a loose wire in my engine bay. How do I figure out where it goes?

I just purchased the truck and it seemed to run good for the first few days.
when I started looking around in the engine bay I have found a few wires that
seemed to have rattled loose and become disconnected. All except 2 wires
have obvious placement when looking at the wiring diagrams. The 2 left are
from the main fuse block running along firewall in the engine bay to the
passenger side of the vehicle. Wires were sitting on the frame. Truck starts and
runs but seems to be missing like it has a bad plug. I have replaced the plugs,
battery, radiator, brake lines, and hoses so far. I have not disconnected any
wiring in the process though.