Hi, In addition to the relay coil resistance, it has significant inductance.
"An inductance acts to oppose a change in current in a circuit". So the current in the relay coil will actually be lower at the moment voltage is applied, and rise to the value predicted by the resistance shortly thereafter (See http://en.wikipedia.org/wiki/Time_constant#Time_constants_in_electrical_circuits
if you want gory details) .
But in the real world of automobile vibration, temperature extremes, flying salt slush etc. you may want to consider what happens if there is a fault
in the relay or wiring. Almost every component in a vehicle has a fuse to protect the system if Bad Stuff Happens. Your relay (and anything else you add) should be protected by a fuse or circuit breaker in case of short circuits or component failures. You may be able to tap off one of the many fused circuits in your vehicle, or add you own, as an "Inline Fuse" (See Car Parts Store or Radio Shack).
Heavy current devices in a vehicle, like the alternator, starter solenoid, etc. are often protected by "Fusible Links", which are short pieces of thinner wire that will melt under a serious fault condition. Usually the only thing that has no intentional short circuit protection is the starter motor.
You can try this with a 2 inch piece of #22 or so wire, connected in the middle of a large (say #12 or larger) wire. Try it out with Mr. Screwdriver AND gloves!
Lots of relay How-To here: http://arduino-info.wikispaces.com/ArduinoPower