I want to be able to run multiple HUs in my car but I'm not 100% sure how to do it. I want the ability to play mp3s but the HU I currently have doesn't have the capability. I don't want to get rid of it either because I'd have to spend a decent amount of money to get something with similar sound quality. What I would like to do is keep my current HU but get something like an Alpine HU with mp3 capability and run it alongside the older unit. I'm just not sure what kind of equipment I need to get this working. I know Audio Control has the 4.1 which has dual source inputs but is this the only way to go? What do I need to get this working?