The advice to make or buy a LRU interconnect diagram is spot on. I chose to pay Steinair to do mine. All my components were purchased through Steinair, so they knew exactly what the configuration would be. (all Garmin).
Now...to answer you question on how to figure out the route and distance for wiring, here's what I did.
I used some small diameter rope and snaked it through the fuselage. This is how I figured out the route. Anywhere there was a branch to a LRU, I taped some parachute cord to the main rope and marked it with the LRU name. All the twists and turns were simulated with the rope and cord, all the service loops were determined and precise lengths were measured.
This harness was built on a table and then installed in the fuselage. All the termination that could be completed were. The terminations that had to go through grommets and small openings were terminated after installing the harness in the fuselage but all the pins were already crimped. One could probably use the rope and cord method but build the harness in the fuselage but that seems harder to me. I also did a triple continuity check before installing the harness in the fuselage...I also made a pinout book.
Here is a
link of a very boring but informative video to the rope and cord in the fuselage.
Here are photos of the process.