I disagree.danregs said:
In order to put the lighting circuit under the correct load, you need a small enough resistance to generate a large enough current. But if you put a small enough resistance in series with an LED, the current will burn out the LED. You simply can't put all that load current through the LED because it'll kill the LED.
The LED bulbs therefore have a moderately large internal resistor in series with the LEDs. There's no getting away from that. If they didn't, the LEDs would die in a fraction of a second - you can't just put 12V across an LED and expect it to survive.
And this is where the confusion lies - I'm convinced we're talking at cross-purposes. The LEDs themselves have a tiny resistance: I agree. But the LED bulb assembly has a fairly large resistance due to its built-in series resistor. That resistor means that it puts the lighting circuit under a tiny load.
The only way to increase the load on the lighting circuit is to wire up a load resistor ACROSS the LED bulb's contacts (i.e. in parallel). You'd choose a resistor which is substantially less than the LED bulb's internal resistance. This creates a larger current that bypasses the LED bulb, thus satisfying the expected load of the lighting circuit.
By my reckoning, the internal series resistance of the LED bulb muddyboots mentioned is about 130 ohms. I reckon you'd need a load resistor of about 7 ohms (give or take) across that to create enough load to fool the lighting circuit into believing there's a conventional bulb fitted.
