what you need is a wiring diagram for your vehicle.
That will show you how to interface to the cars circuitry.
Sometimes it requires a relay, other times you can get away with simple transistor,
it all depends on the cars circuits.
Sometimes you can find it on line, other times you may have to order it from
the dealer, which can be expensive, sometimes over $100 for the full manual.
Sometimes you can buddy up to one of the guys at the dealership in the shop
or parts department and they can give copies of the few pages you need.
--- bill