.......If you increase the resistance it's like closing off a water tap. The flow becomes restricted and less water gets through so there is less to be used....
So, increase the resistance and less electrictricery is there for the LEDs to share. So they glow dimmer 
(this only works to an extent with LEDs though, as they're not quite like incandescent bulbs!) You may however need to find some lower mcd LEDs? This requires experimentation!!!
So personally I'd get maybe 10 soldered up and using the calculator in the above link work out the correct resistance. Then I'd increase it by 2 and see what dimming effect it has in the array. etc, etc till I had the desired effect.
Then I'd make sure my resistors were up to the job... 1/2 watt or possibly higher to handle the load/heat and then make my lights!
............
Guys,
In this environment, I think you may not need resistors, or at least you can manage them downwards :
An LED has a pretty much fixed operating voltage (the "forward" voltage) and then a maximum current.
Assuming you're just using relatively steady DC, you should not exceed the forward voltage. You can operate at a lower voltage, but the output falls off very quickly.
Classic use for a resistor is if, say, you want to run one LED off a 12v supply. Your LED specs will give the forward voltage - say 1.7v - so you need to make sure your LED
only sees that 1.7v - no more - so you need to 'drop' 10.3 volts. Your LED spec will then say what the normal operating current is - say 30 mA - from that you can calculate both the resistance and the power rating required. Always have a higher power rating resistor than the power being dissipated to allow a safety margin; especially as running on the limit in a vibrating environment such as a vehicle makes failure more likely.
Now, in this example above, we're throwing 10.3/12 ( = 86% ) of the power away as heat. If we only want one LED (and don't want to indulge in any fancier PWM electronics) then that waste is the only way. However - if you're going to have multiple LEDs, why buy power resistors and have a hot-spot in the car / sidecar etc.? - Instead of wiring say 10 LEDs in parallel with whopping resistor(s), use the LEDs themselves. Say you were planning on having 10 LEDs - you could start by wiring them as two strings of 5 - i.e. two separate batches, each of which has 5 LEDs in series - that way, if the forward voltage of each LED is 1.7v your string now has a forward voltage of 8.5v and you only have to drop 3.5v
There is a small danger that if you put all the LEDs in one string you have a single point of failure (who can remember hunting down which bulb had blown in the Christmas tree string) - but LEDs are a lot more reliable (if not over-driven) - so maybe re-arrange your LEDs so you have them in a minimum of two strings;
- and if you're canny, you can arrange that each composite string has a forward voltage of just over 12v ! - i.e. no resistor required!
(In an auto environment, you also have to accomodate varying voltages - your lights need to be bright enough at minimum voltage, but you need to make sure the LED won't be over-volted at the highest charging voltage from the alternator).
Also - saw mention of 30 degree viewing angle. It's not a cut-off, but it does drop off significantly beyond the angle specified. Brake lights may not need a tremendously wide spread, but turn signals may need more than 30 degrees, so you may have to have more LEDs on a convex mounting.
Dave
