The (simpler) alternative - does your existing HU have 4V Line In capability?

I'm assuming it doesn't, or else it would have been hooked up that way to begin with. I seem to recall he said it was a factory system, and most factory systems don't have line-ins.

One thing the installer may not have thought of is to hijack the CD-changer inputs and route the empeg in through those "as if" they were a line-in. Sometimes that can work with the right trickery. So that's another option.