A:AnswerBased on the KDC-X599 looks as though its from 2015 and the KMM-BT328U is a 2020 model. The harnesses are not the same. You would need to use the harness that comes with the new unit when doing the installation.
A:AnswerNo,
The faceplate is just the controlling buttons etc. It is the installed piece that is the actual unit. You have to change the entire head unit.
A:AnswerIf you have one set (lets say your fronts) wired to speakers inside the camper you could use the leads for the rears for the outside speakers. Or you could also run amplifiers off of the rca preouts and wire the amplifiers to the speakers. I would just make sure that you use appropriate speakers for the weather (you'll probably want to look at marine grade ones if they'll be outside a lot) and ensure that your connections are all heatshrunk to ensure that no water gets in.
