I've got a Mitsubishi Montero (with the factory Infinity Stereo). I know the problem your having. This is what I found.

You'll find that most people will tell you have two options.

1. Connect your empeg to the infinity using an FM modulator. You'll lose some sound quality, as the empeg will be playing through your radio.
2. Yank out your entire system (including speakers) and replace everything with an aftermarket amp that has RCA inputs. Or, a new headunit that that has AUX inputs or outputs.

A handfull of other people have approached this somewhat differently. One of the users on this board has a Dodge Durango (that has/had the Infinity system). He used something called a PAC-OEM to attach his empeg to the existing infinity amp, and ditched the head unit.

I'm seeking to just add the empeg to my system. I like listening to the radio, or playing a childs cassette for my kids, or even listening to a cd or two.

I ordered two pieces of equipment so far (neither has arrived yet). The first, is something called a MITAH. Its typically used to allow you to attach an aftermarket head unit to a factory amp. This one is specially made for the Montero/Infinity system. You'll find that the Infinity head unit is connected to the factory amp with a 6-pin din cable. Most head units have RCA plugs.

The MITAH will allow you connect to something called an RCA switcher. This little device lets you connect TWO head units (the Infinity and the empeg) to a single amp.

Past this I have two challenges left. The first is to cobble together a cable to connect the Infinity to the switcher and somekind of "Y" type wiring harness to connect both units to the car's 12v constant, and other wires.

I'll keep you posted on my progress.