You'd want to use a radio/head unit to connect to the speakers. You'd connect your iPhone to the head unit. If you want to use just the iPhone, you can use an amp instead of a headunit/radio. You use an RCA/headphone jack y cable to connect the iPhone to the amp, the amp powers the speakers. Do you have a 12v/cigarette lighter plug? There is a soundbar that connects to a 12v outlet and an iPhone. It's low powered though, you'd be better off with any soundbar and an amp.
iPhone app user; excuse typos...thanx.