Hi,
I've attached a picture of the bumper car cakes they made on Ace of Cakes, I think basically you need a ball of fondant, flatten the bottom a bit for the bottom of the car, make a little cut out for where the person would sit, two little dots for the headlights, and a little square piece of fondant for a grill on the front. You could complete it with a little piece of wire, and a fondant ball on the wire and stick it in the back of the bumper car.
http://www.google.co.uk/imgres?q=ace+of+cakes+bumper+car&um=1&hl=en&sa=N&biw=1280&bih=603&tbm=isch&tbnid=MotWT_Nae_rtZM:&imgrefurl=http://www.dailyherald.com/article/20100407/entlife/304079973/&docid=T_iDctNLSzdV9M&w=248&h=186&ei=7aNGTpTCLcLBhAeX8Z3IBg&zoom=1&iact=rc&dur=407&page=1&tbnh=142&tbnw=195&start=0&ndsp=16&ved=1t:429,r:1,s:0&tx=154&ty=54
Hope this helps, good luck! 
Quote by @%username% on %date%
%body%