In the simplest terms, yes. Aftermarket head units generally have RCA outputs (like your home stereo) that are meant to be run directly to the input of an amp. So signal cables get routed to wherever the amps are mounted, then speaker wire is routed to the speakers. Most of the complexity (and there isn't much) comes from figuring out what you need, what you have, and what you want to do with it all. So that is basically just figuring out what amps to get based on the speakers that you have, and how to supply those amp(s) with signals from the head unit. The rest is just running wires.