Gathering the parts

The parts you'll need to modify the converter box include:

• 4-pin header (standard 100-mil spaced)
• 12V cigarette lighter jack with cable
• various sizes of heat-shrink tubing

The 4-pin header should be bent so the outer two pins are touching and the inner two pins are touching as shown.  

For a 12V cigarette lighter jack I chose the MPD ZA5077 available from Digi-Key for $3.74/each.  The Digi-Key part number is ZA5077-ND.