Hmm, could always try using an in-dash EQ. an EQ will have RCA jacks for front and rear speakers as well as a fader knob and remote wire. Check out this thread for more details. Wiring - EQ,opus,4ch amp,subs,lcd-drawing idea
I dont really know of a unit that is just a pre-amp and flip out lcd display. Using an EQ is like having a HU without all the bells and whistles. Hope this helps.