I know you can buy ceramic car heaters that plug into the lighter socket - wouldn't give light but ??may provide background heat if it could be rigged up straight to the battery but not sure how quick it would drain it etc as I am useless with this sort of thing but sometimes see gadgets that catch my eye!!
Here is another thought if using a greenhouse:
http://www.reuk.co.uk/Solar-Greenhouse-Heat-Sink.htm