Hi Paul,
its not a big deal with the leds! You didn't watch the voltage of a single led. So if you run a 12V system and want to use 3.2V leds you can only put (12V/3.2V=3.75) three together and still need a resistor for the remaining (12V-(3.2Vx3)=2.4V).
You have 12V and these 12V have to be shared by all the leds you put in one line. If one led gets more than the optimal voltage which might be between 2.somethingV and 3.somethingV it won't work for too long. If it has to share the 12V with to many and gets less than the optimal voltage it won't be as bright as it should be or won't work at all.
And for the case that you have 12V and 4 single leds with 3V each, you still need a tiny resistor as leds should never be used without one.
You find calculators on the web:
http://led.linear1.org/led.wiz
When you work it out keep in mind that the actual voltage of a fully charged 12V battery is more than 12V. So if you work out the number of leds you can work with 12V but if you want to calculate the size of the resistor, better work with 13V.
Just let me know if you need help.
There are alternative systems. It only depends on how much you want to spend. Tight a... solutions start at a few dollars for manual switches. If you mind bending your knees you can get switches for RC use. I got a "servonaut" smx sound in my Volvo, but thats nearly 300 bucks. Speaker goes extra
http://www.youtube.com/watch?v=s8AG7jNNHRo
...not my scania...