ok mate some thoughts. 
  
- Your going to need to know the best temperature to keep the bottles at and also the max surface temperature they can handle. 
  
- the easiest option is to get some heated seat elements out of something from the scrappies.  
  
- if you wanted, you could even get a spare "elbow operated" (heated seat) switch and put it next to your heated seat switches to control the bottle heater? 
  
- you would need to find some kind of insulation that would be happy with the heat levels from the element & would be effective at keeping the bottles warm. 
  
- possibility is to have a thermostat and be temperature controlled to; not waste power, extend the life expectancy of the elements, keep the perfect temperature & stop you from having to turn it off and on.