There is a better way that will end up with less of a cable cluster and is easier to hide with less junk and does not leave you with a ton of wires and connections. I took the multi adapter that came with my mogul. The one with the mini usb b socket and the mini 2.5 mm female jack on it.
I cut the 2.5mm jack off and i wired it up to the wires from the stock htc headphones, by cutting off the htc ext usb end that comes on the stock headphones.
On the other end of the adapter where the ear buds are after the mic i cut the buds off exposing the audio out signal thats just left + right + ground and anothe un needed ground.
I then just wired up and taped all the connections the result was 1 small adapter that I power with a usb cable and audio oes into the back of my headunit. I extended the wires to the mic to up by my visor so i still can use hands free if someone calls without having to disconnect it from the touch pro to take a call.
A step by step for making my adapter from spare HTC headphones and the splitter adapter can be found here.
http://forum.ppcgeeks.com/showthread.php?t=49137
and a youtube video from back when i was using it with my mogul. Still works with my touch pro will work with the touch pro 2 as well and any other HTC device that uses the 11pin EXT usb socket.
http://www.youtube.com/watch?v=CfepHUE1EYY