Before you guys go to ordering high-priced voltage regulators, you need to check this little unit. Input up to 35 volts, output 5 volts.
http://www.radioshack.com/product/index.jsp?productId=2062599
You also need a filter condenser to go with it. I'll look later (tomorrow) to see just exactly which one to purchase.
Better yet, check this out.
http://www.moparts.org/Tech/Archive/elec/11.html