There's a post in rec.aviation.ifr titled "Garmin GMA 340 Music Input"
that I found by accident while trying to find the melee I got caught up
in. In it someone is describing using transformers (from "RatShack" no
less) to impedance match a line level output to his Garmin 340 input.
Apparently it worked out for that guy. So if you haven't selected and
biased your transistors yet, check it out.