I guess you are wanting to hard wire the power. If I recall correctly it uses 2 x AAA cells so you will need to have a regulator to reduce your 12-14V ship supply to 3V. This is not rocket science, an electonic hobby store could make one for you quickly, the current draw is low.
As an aside, how long does the S100 run for on alkaline batteries?
As an alternative you could look at the Sanyo 'eneloop' ni-MH batteries. All the blurb suggests they have a flatter discharge curve and do not self-discharge as much as other types. They are 800mAh. I just bought some but have not had enough use to give any comment. They can be recharged in a normal NiMH charger.