Well, as the formula states, V = IR. Since volts will always be 12 on a car, then you have I = 12/R. If you have 8 ohm resistors, then you are using 1.5 amps.
Power (watts) is simply current times voltage. P = IV. You can also use P = I^2 * R. So, power is 1.5*12 or 1.5^2 * 8. That's 18 watts.
If you run the resistors in series they just add up. If you route them in parallel, then it gets a bit more complicated. You have to square each resistor's value, then invert them, then add them, then take the square root.