Some of the simpler electrical stuff these days can be added afterwards, like Brake Controllers, Bed lights and so on.
Just added the RAM bed lights that came out in 2016, the only thing missing from the truck I bought off the lot. Easy to do it yourself if your truck is a 2016 as all the wiring is right there. But then you have to take the truck in to get the truck's computer updated or the stupid lights don't work. That was another C$50.
Similar computer update needed when adding the OEM brake controller I'm told.
What RAM does is first update your vehicle configuration in some central database, then use that to upgrade your truck's configuration. In my case, the dealer was able to do the first bit, but he didn't dare upgrade my truck's computer because their internet was acting up. Apparently if the update fails your truck is dead. So I had to come back later to get it done. Monumental hassle.