why dont you buy a cheap battery operated programmable room stat and feed the motor from the car battery via that?
like one of these
http://www.plumbworld.co.uk/glowarm-rt500-programmable-1932-17046?CAWELAID=205025076
or
http://www.screwfix.com/prods/17585/Plumbing/Central-Heating-Controls/Horstmann-DRT2-Room-Thermostat
matt
(edit)If you really need it to be on for just seconds you will need something like here
http://www.rayleigh.co.uk/product_results.cfm?category=1&subcategory=22
like one of these
http://www.plumbworld.co.uk/glowarm-rt500-programmable-1932-17046?CAWELAID=205025076
or
http://www.screwfix.com/prods/17585/Plumbing/Central-Heating-Controls/Horstmann-DRT2-Room-Thermostat
matt
(edit)If you really need it to be on for just seconds you will need something like here
http://www.rayleigh.co.uk/product_results.cfm?category=1&subcategory=22