It looks like Blitzsafe offers an AUX input adapter for Chrysler, but I'll assume he didn't know about that.

Instead I'm going to speculate that he ran the factory head unit through a line-level converter and then into the AUX input on the Empeg. Only down side to that is that the radio and tape don't work without the empeg in.

EDIT: Nevermind...he posted what he did below.