Advice on driving the DRL leds
A simple solution for raw diodes can be the following schematic:


Credit for this wonderful image goes to Brian Duxbury. He has a nice post about this schematic here.

You regulate the current with the sense resistor, the approximate formula is this:
$$R_{sense} = V_{base-emitter}/I_{diode}$$ For ballpark estimates, expect the V_base-emitter to be 0.7V, so if you want 20mA, that's 0.7/0.02 = 35 Ohms. 

Some notes for choice of transistor


the "Drive" transistor  has to have a maximum collector-emitter voltage exceeding the maximum expected voltage of the power supply (i.e. if the power supply max voltage is, say 16V, better be on the safe side and take a transistor with a maximum collector-emitter voltage of at least 20V)
same goes for collector-base voltage rating
consider the voltage that'll fall on the transistor and thus, calculate the power requirements (i.e. if the LED current is 20mA, the supply voltage is 12V and the LED's forward voltage drop is 3V and there's three LED's, it means the transistor must be able to dissipate at least $$(12-3*3)*(20*10^-3) = 60mW$$


A nice feature of this schematic is that you can control it with PWM via the control pin, so if you know how to generate such a signal, you can just attach the generator to the control pin and off you go. R_control can be around 10K or so. 

For multiple strings of LEDs, you can put such a constant current generator on the bottom of each of them (SMD parts would make it a very compact device) and just join the control pins at the PWM signal source to adjust light simultaneously for all strings.



If the LEDs already have resistors, if you can find out the value of the resistor you can calculate the danger of the LEDs suffering from overcurrent (you can find common forward LED voltage drops by colour online) and determine whether you even need such a constant current driver 
$$(V_{supply max}-V_{forward})/R=I_{diode max}$$

If you find that the LEDs are sufficiently regulated by the resistor, you can simply insert a MOSFET between the LED and the power supply (correct orientation of course) and use a PWM signal on that to control the brightness.