I don't have any feedback from these guys but let me know if you go through with it I would be iterested to hear how it works.
http://www.abbott-tach.com/cablex.htm
If that is not what you are looking for ask the guys at this website.
They do cable to electronic adaptions, they may be able to assist you with your project. hope this helps.
http://www.tciauto.com/electronics/
BTW why are you wanting to put a thm400 in your truck?