I think the way it could be decided is by first looking at cool vs. uncool and then the two groups in the winning category.
Say we get 3 subzero, 4 cool, 4 uncool and 1 seriously uncool. We would see that there are 7 votes on the cool side and 5 on the uncool. So then we see cool has one more vote than subzero, so the car is cool. If it was a tie, it would go in-between.
The problem is that on Top Gear it worked kind of as a scale, so even under one category one car could be cooler than the other. The best way to replicate this would probably be with a scale like Dragonistic's but where each member picks a number inside that scale. But that would be a bit too complicated.