what resistor to dim 12v led bar

The resistor needs to go in series with the load.  You are going to have to drop the voltage a lot to get them to dim even a little.  Do you have an electronic parts retailer near you?  If so, Have the wires accessible when you drive there, purchase a package of 1K ohm 2 watt resistors.  Wire the resistor in series and see if it dims the unit to your liking.  if it does not, wire the second 1K resistor in series also, test again.  If the 1K resistor makes it too dim, parallel the second 1K resistor with the first one to give you a 500 ohm load, turn them on and test again. you will now know how bright they are with 1K, 2K and 500 ohms worth of resistance.  You should at least have a better idea of what values to try next. 

The resistors come 2 per pack and sell for 1.00 in the U.S.