I would NEVER use solid copper wire in a mobile application- too much vibration, and a single break could lead to system failure or intermittent. Stranded copper is much more flexible AND single fault tolerant (a couple of fine strands break here or there and the conductor is perfectly fine for many years.)

Do agree on the tekonsha prodigy- use it to control a 14k lb 3 horse trailer I pull with another vehicle

A