No, the factory radio doesn't give you any extra inputs in the back for anything like that. About the only unused input it has is one for a 6-disc CD changer, which Chrysler never even offered as an option. (The German company, Becker, who made the radio sold one over in Germany that's compatible - so a FEW people got ahold of them through eBay or what-not and used them with their Crossfires.)

I know I've seen devices in the past that work like FM modulators, except they have a T connector that allows attaching them between the radio's antenna jack and the antenna plug, so the signal is injected right in with the least amount of interference.

The factory amp isn't going to have any spare connectors on it for what you want either. People have been cutting the signal input wires and splicing RCA jacks onto them so line-level outputs from aftermarket stereos can be run directly into them. But you can't just splice 2 pairs onto one set of wires and expect 2 audio sources to share the amp. The signals will cancel each other out and overload the input....

What you *could* do, I suppose, is to build a simple mixer circuit that accepts 2 sets of RCA line level inputs and send the mixed/combined signal to the amp. There are actually diagrams on the net telling you how to build one, because others have done this before.... but you have to know a little bit about electronics.