Car stereo/amp question

I'm putting a smallish (200x2) amp in my car Real Soon Now. The factory head unit can drive the speakers well enough to sound good enough for background music or an audiobook, but when I really want to play music it sounds not awesome. Better than a clock radio from 1992 but not by much.

The thing is I want it both ways. When I'm playing an audio book I don't need the amp and want it to play directly without the amp in the speaker circuit at all.

This isn't something beyond my ability to solve, I could knock out a nice solution with relays and blinky lights and whatnot to do the job triggered by the antenna/amp line like the amp would be, then switch that from the dash. But if there's an existing solution that isn't stupid expensive I'd rather not reinvent the wheel.

Has anyone done this or am I the only one who would even want it?

One more important point: the amp will be using line level input, I'm going to install RCA lines for the future but the factory stereo has no low level outputs. It would be dead easy to do what I want if it did, but alas.