Yes, it is a pain to crawl under and check the wiring.
My first experience with crappy factory wiring was back in 2007 when I serviced the bearings and adjusted the brakes on my toy hauler. I found one brake out of the four that had never worked since day 1, and that was after I'd owned the rig for 4 years. The wires were assembled with crimp connectors, but were not making proper connection.
Another trailer I bought new, I checked everything before making the first trip. It was a flatbed car hauler. It used crimp connectors, but not the heat shrink style, so water could get in. If I were to tow during the winter, when deicer is used on the road, corrosion would surely wreak havoc.
This was a flatbed car hauler for my Jeep.
The crazy thing is, not the same style crimp connectors were used at all wheel positions.
One brake assembly was this:

Another one was like this:

After my upgrade:
