I have all aftermarket equip in my car now. I have line-level in the
dash and an amp in the trunk.

I am now trying to mount a portable garmin gps unit in the dash
semi-perm style. it has voice prompting via a 1/8" jack for output.

my problem is to find a way to mix that audio with the head unit
audio. I don't want to degrade the head audio and I bet there is some
noise in the gps output. so maybe something like a VOX circuit that
detects audio from the gps and mutes the head audio (like a cellphone
circuit would), lets the gps speak its directions, then after a
timeout it switches back to main audio.

are there things I can buy that help with this? ideas on how to do
it?

all I have is audio out from the garmin and audio at line level from
the car stereo. no mute lines, not even really a head unit since I'm
just running some mp3 output almost directly into my amp (but using an
audiocontrol as a front-end).

ideas?