Hi Eric,
I really dont know how the lights are set up and well expect they have a common earth to chassi.
If this is correct than it will be required to switch 12v+.
If this is the case, a high side switch will be required, [ as per the FET dwg]
I was thinking collector to 12v+, emitter to led light, and a resistor between base and picaxe.
The max voltage across the LED would be 5V-Vbe drop, if a darlington was used the Vbe would be ~ 1.4V, si if I follow your description correctly, all you would get across the led would be 3.6V
I also dont know how the leds are configured in each light but would expect only a supply of around 2 volts through the light, so i would expect the transistor to switch on fully as the base voltage (5v) is higher than the load voltage required.
As its an emitter follower the BD transistor would not saturate.
Correct me if i am thinking wrong here.
Pete.