If your lik eme, you always looking for more ways to use your kit. I boought the TTN3 and found the audio output pretty poor. I almost bought a powered speaker car kit until i came across this rather nifty device.
http://www.handnav.co.uk/product_details.php?id=34
This device will tramsit the audio output of your PDA into your radio on 1 of 8 predetermined freq. now whilst im driving i play all my MP3s through the radio at the same time as the voice commands from the GPS. and the audio is pretty darn good. the only time i have lost a bit of quality was when i was driving around london with all the radio traffic leaking across the bandwidths. i could still hear the music and GPS but it wasnt as clear as driving outside of london. If you do buy this, you may find that on first trying it that the audio sounds distorted. i have found that by turning the audio on my ipaq down to the first notch and allowing the car to raise the volume the sound was crystal clear.
Just set the output volume of you PDA, set your Media player output to around 2/3 of the way and adjust your GPS voice output to be louder than the media player. now you can play MP3s and listen to the GPS commands at the same time and control the volume ouput from the car stereo.
I bought this along with a brody car mount that took less than 5 minutes to install with any holes to drill.