I have converted my truck to 12V. The heater motor is still 6V. I wired in a voltage converter. I had the converter left over from a previous project. The one I installed is similar to the one listed below.

http://www.ebay.com/itm/48-55-F1-6V...h=item3cd9d8f78b&vxp=mtr#ht_63wt_986