How about you just buy one?
http://www.accessorypost.com/touchcarkit.html
(Whom I ordered from)
or:
http://www.wirelessground.com/touchcarmount.html
It powers the touch and provides a speakerphone....also a 2.5 mm audio out jack to go into my stereo.
3 issues:
1. The audio out seems to be audio limited to protect the small handsfree speaker - read little bass output
2. The cradle must be lifted of of the mounting arm in one hand and then insert the phone with the other (not a big deal, just hang it on the arm when done - when it was new 1/2 the time the insertion would cause a soft reset on the phone - though it has been a long time since it has done it now - I use it every day.
3. Will not work with any extended battery.