i've put in a few of these clarion units, as light as you're going to find i think, it has a built in amp but the bulk of weight in radios is the cd mechs.......it has a 2 volt preamp output.
http://www.clarion.com/us/en/products/2008/audio/source_units/bluetooth_sd/index.html
for a while in my 911 i was using an audio control line driver and my sirius stiletto only, it worked well but lacked any tone control.....
alpine also has a few units...but i haven't played with them
http://www.alpine-usa.com/US-en/products/series-c.php?category=IPOD