[dsraven]

Insulate harness with a rubber grommet anywhere it goes through a panel etc
Use wire loom on any exposed harness
Use clamps to keep harness retained tightly to frame etc
Use a non insulated crimp connector with double wall shrink tube whenever a connection is needed
Try to stagger multiple connections so as not to have a lump in the harness
Use a junction box where wiring terminates at the rear and splits off to tail lights etc.
A flat trailer plug connector from the box wiring and a matching end connecting to the junction box makes box removal easy later. Or simply remove the cover from the junction box when needed
Industrial electrical connections have a compression style fitting that can be used on a junction box and then when wiring is passed through the compression part can be tightened up to seal weather out
Try to use a sealed beam sort of light if possible that has a plug in rather than wiring up to a light socket. They are more reliable for shock absorption and less finicky as far as actual connection to the bulb