From what I can tell from your description, the head unit is directly supplying the amplification for the dash speakers, and there's no separate amp hooked to the dash speakers at all.

Because the empeg is somehow using the EQ as the switcher, instead of using some sort of aux-input on the head unit, and the EQ doesn't drive the dash speakers (not sure but that's what your description sounds like), then that's why the empeg doesn't drive the dash speakers.

Sounds like this whole thing is wired kind of rube-goldberg instead of properly routed with line-in and line-out connections. It's no surprise that you're having odd problems with it.

If it were me, here's how I'd do it, from scratch:

- I'd make sure to somehow have an external amp driving all six speakers, instead of just the back four. Even if it means purchasing a whole new amp.
- I'd get rid of the equalizer completely.
- I'd run the head unit into the empeg's aux-input. Either directly, if it had line-outs, or through a linelevel converter if it didn't.
- I'd connect the empeg to the external amps using all of its line-outs, properly.

After this is done, the empeg becomes the "master" and the head unit becomes its aux input, and you select the source from the empeg's front panel. Note that the empeg's 20-band parametric digital equalizer becomes the system equalizer now, which is why you don't need that original 6-band EQ any more.

You could also run both units into a line-level switcher if you liked. More details here.