Infinity stereos

Just came from the salvage yard for a new radio for my '89 Caravan c/v. They had two am/fm casette units, pretty much identical faces except one said Infinity and the other didn't. They both had the same plug-in receptacles in back, but the Infinity one had an additional, round-shaped jack in back, kind of looked like a DIN connection. What IS that? Something to do with a remote amplifier it's supposed to be used with? If that's what it is, where is the amp located on the vehicle...? etc. I'm sure someone out there knows all about this. Any help appreciated. TIA