Does this dealership have an know how to use a smoke machine? It really is the best/only way to identify Evap leaks. As for having a bad harness, well this is an electrical system and we always explain that to our customers. It can always be a mechanical or electrical issue. Not sure it would need an entire new harness, other than it will save labor hours to replace it all rather than isolate the issue and repair it. My suggestion is find a better place to take your truck to for repairs. Dealerships want easy profitable repairs, not necessarily the time consuming diagnostic and electrical repair stuff. BTW the wiggle test is pretty common diagnostic approach these days. You would be surprised how often it works.

Oh yeah FWIW the last one we had actually had a crack in a rubber grommet that was the issue. The evap line had a rubber connection from the one of the plastic lines that had dried out and cracked. We found it with a smoke machine. After it had been to a shop without one that just kept throwing parts at it. A lot of shop tend the throw parts at evap leaks because they don't want to invest in a smoke machine.