I made a lead up 2 years ago no problem, I used a Mobile phone charger to drop the 12V Boat / Car supply to 6V for the Garman 12. The plug and wiring diagram came from here: http://www.pfranc.com/projects/g45contr/assemb.htm
Why not buy the cable, which is readily available and includes a 12V to 6V cigar lighter connector so you have the option of running the Garmin from external power.