LEDs like that typically run between 5 and 50 mA, though they could be higher or lower. If you don't have specs on the parts you have, a good starting point is about 10 mA, and you can adjust based on how bright it is and how bright you want it.
Then you can calculate the resistor from that using Ohm's law, V = I*R, where V is the voltage across the resistor in volts, I is the current through it in amps, and R is the resistance in ohms.
Assuming you want 10 mA (that's milli-Amps, or 1/1000ths of an Amp), I=10/1000 = 0.01A.
When running, a car is typically closer to 13.8V, and when not running closer to 12.8, but it varies with the car and the conditions. I'd base calculations on 13V.
The value of V you want for the equation is the voltage across the resistor, which is the car voltage minus the LED voltage, or 13V minus 2V, or 11V. You can do the calculation for the 3.5V LEDs.
Rearranging Ohm's law, we can get the equivalent equation R = V/I. Since V is 11V and I is 0.01A, R = 11/0.01 = 1100 ohms.
As it turns out, that is a readily available value. Not as common as 1000 ohms, but if you are purchasing online, it should be available. If you are looking through a junk drawer, 1000 is a more likely find. When purchasing, these values might be listed as 1.1K and 1K, respectively. The K stands for Kilo-ohms.
Don't get hung up on getting exactly the right value. +/- 10% would be barely noticable in the brightness.
