Here is how I did it.
I have to preface this tutorial with the fact that I have access to a 3D printer for some of the parts I made. If you like I can provide 3D files but I cannot make more parts for your projects.
Step 1: What I got to Start
I took the IH13 totally apart to extract
5way controller board
5way controller rubber cover
2 controller boards
and the iPod connector
Then I took the Kensington car mount and removed the MP3 player clamp off of the bendable arm by removing one small screw.
Step 2: Design the power regulator
I had originally wanted to have a lighter adapter for a GPS unit also but decided against it in the end (I am hoping the new iPhone will have GPS ).
Most parts were obtained at Radioshack. The LM350 variable regulator was purchased from www.digikey.com because I needed a regulator that would supply 2A or more.
Step 3: Design the custom parts
STL files are available here: VespaSpeakerSLT.zip