If you are getting a little more voltage than is acceptable (My brother's gets 14.4v no problem) then you could try having a diode in place to provide a .7v drop. Or 2 for 1.4v drop. Of course this affects running the pc with the engine off too... If you need it working fast, this may work tho until you figure out just how you want to proceed.
It may be that you get a 12v regulator, or a diff. psu like the opus.