Just use the Hamsar unit from Canadian Tire. It claims not to be compatible with manufacturers like Subaru and others but the instructions inside show you what to do for Subaru and others.

I recommend this over any other method especially direct wiring and other tricks as you could cause serious problems for yourself if you mess it up.

The Hamsar unit is a cheap and reliable solution with little risk compared to direct wiring.

http://www.canadiantire.ca/browse/pr...08474396673462