Yeah looks like you need an "add on" voltage regulator. Otherwise you could retain the stock ecu for that function but you're kinda outta luck on that front. Maybe you could find an ecu from a junkyard neon or something? I have a feeling that a separate voltage reg from another mitsubishi like John said is probably the cheapest, most straight forward solution though.

Oh and I dont know about anyone else, but I would not run that car at all with it charging upwards of 18v. Especially with your new shit all wired in.