I am a little bored at work right now, so Here it goes
There are two ways to vary the output of an LED. The main thing you have to remember is that an LED is a Diode. AS such, it is a one-way PN gate
umm.. in laymans terms, it only allows current to flow in one direction. Applying current in the other direction has no effect, and in fact, is electrically disconnected.
The point of this is that a LED is a Direct Current Active device. As such, if you simply require a two state mode, IE, full on and 1/2 on, you CAN simply use a resistor and a shottky diode to create a dual voltage to the LED. The advantage of the LED over a light bulb is that the LED draws SIGNIFICANTLY less current, and therefore uses a very small resistor. These resistors can be as small as a grain of sand, and do not get hot or even warm.
The intensity of the LED can be adjusted by simple current limiting (changing resisitor value, or using a small potentiometer).
The second way is a bit more complex, but results in the ability to do TRUE fade in/out, multi color.
This method requires a coule of ICs (Integrated Chips, or 'chips') one would generate a clock, and the other generates a 'Pulse'
Uhh, here it gets kinda technical, but I will try to keep it simple..
By changing the shape of the pulse, in this case the width of the pulse, you can 'simulate' any voltage between 0 and full input voltage (12V in this case) How it works exactly is not important, what is important is that the LED (or in this case it would be the resistor network) 'sees' a particular voltage based on the width of the voltage 'pulse'.
This allows you to infinately (not really, but virtually) adjust the voltage to any component, in this case an LED. Thus the intensity varies with the 'seen' voltage.
Is everyone batty yet?
The advantage of this system is that with the proper chips, you can FADE LEDs very nicely, and in fact can FADE different LEDs in the same housing to give the APPEARANCE of slowly chaging colors. This is a GREAT effect, and one of my personal favorites.
Now that everyone is clear
I would be using the simple method for dual mode side marker lights. The second method is really if you wanted something that is really showy, as it wouldnt be terribly useful as a blinker, but it could cycle color while it is on. So it would be on with the parking light, but could cycle colors.
I hope that cleared it all up.
, But I had one stop working after about 15 Hours of run time. That Pi**ed me off, I returned them about 2 months ago and still no word. 
Iraq-Land of the lost!
