I would use the 12V supply to power them - you place several in series and add a current limiting resistor - for more LEDs you add more strings of LEDs (plus resistor) in parallel to the first string. It can be quite efficient since most of the battery voltage (and hence power) will go into the LEDs.
However a quick word of caution - you need to make sure they actually light up when the battery is low (brake lights are important!), so you should assume the battery voltage is only 11V or so in your calculations (and then recalculate for 13.8V - battery being fast-charged).
So step 1 is to lookup the forward voltage of your LEDs on the datasheet, preferably the graph of forward voltage against temperature. Vehicles are exposed to a wide range of temperatures and the forward voltage will vary quite a bit. It is wise to actually measure the forward voltage at 70mA too - the datasheet might be "optimistic".
So given the maximum forward voltage and the minimum battery voltage you can work out the number of LEDs in one string. For example I'll assume a forward voltage of 1.9V and min. battery voltage of 11V - suggests a string length of 5 LEDs (taking a max of 9.5V).
Then you need the max current for the LEDs, lets say this is the 70mA you quote. Also assume the battery voltage can go up to 13.8V and the LED forward voltage minimum is 1.7V (off the top of my head, don't trust these values!), then the string will take 8.5V and the difference from battery to string is 5.3V. So series resistor is 5.3V/0.07A = 75 ohm. It will dissipate about 0.4W.
With 30 LEDs thats 6 strings of 5, ie total current of 6 x 0.07A = 0.42A
One MOSFET will happily switch this, so long as its a logic level one - source to ground, gate via 220ohm resistor to Arduino pin (or whatever), drain to the cathode end of the strings. Anode end of strings to +12V (don't forget a 75 ohm resistor in each string.
Finally we can calculate the min current (assume Vbatt = 11V, Vforward=1.9V, then that leaves 1.5V across resistors and 20mA for each string instead of 70mA
An alternative scheme is to use a constant current LED driver chip - this will prevent the variation of current with temperature and battery voltage.
[If you used a 5V regulator you'd be limited to 2 LEDs per string, so 15 strings = 1.05A. The regulator would need to dissipate upto 10W]