Since I at one time attempted to get it going again here are some suggestions:
Limit ROTM forum's access to members that have a) reached a certain post count and b) been a member for X amount of days. Easyily done if I remember correctly. This limits who can vote and such. As you are aware of previous issues where cars would suddenly get 30+ votes because they were behind.
As I listed before narrow the "theme" idea. Hottest hatch, grocery getter, a car related theme. It's been a complaint that it's almost Honda/Acura ROTM every month and that is why previously it was attempted to give each sub section to display to all of TR their top car.
Nominations, first 15 cars nominated which fit that months criteria(yeah more so the voting may/may not be closer) or cut off at a certain date with whom had been submitted.