The resistor divider on the NPN's gate lowers the 4.6 volts bellow 500mv, the base junction never becomes forward biased. If you skip the 10k and 1k resistor divider on the base and just use a 1k resistor to the base it works fine. What's the resistor divider for?
Yeah, I had a brain fart. The base-emitter resistor should be 10k, not 1k. I like base-emtter resistors where the signal might be noisy, or the input pull-down is unknown.
I'll edit my schematic when I get downstairs to my other computer.
hrmm, didnt' notice that (didn't simulate it either) don't worry about modifying yer schematic, the one I modified is still yours even if I posted it =)
Uncle's last schematic should work just fine, given what you've stated so far I don't think the 470 ohm gate resistor is NOT required though, use it at first remove it if it doesn't work. The voltage isn't incredibly sensitive, generic NPN/PNPs will be well within a car voltages range.
See my corrected schematic in post 52.
Sceadwian's circuit in post 54 should work if you reduce the value of the 600 ohm resistor to 100 ohms or less (it is not really necessary, same for the 100 ohms in my circuit). IMHO, it wastes a little power, which probably is not a problem in a car.
Either one should work if you follow the recommendations in my previous post.
Of course, the relay circuit you posted (with the BJT) should have also worked.