Are you trying to detect if the heater element fails? That implies sensing current through the heater, not as simple as detecting the presence/absence of 12V.
A shunt resistor that gives 0.7V or so could be used to turn on a transistor that powers an LED, but the shunt resistor will get warm/hot and reduce the efficiency of the tyre warmer a little. A smaller shunt with a comparator to sense a smaller voltage would be better.
Although I agree that your idea is probably a good solution, I must comment that each of his heaters draws 0.75A (3A overall/4) so a resisistor to drop 0.7V would be around 1 Ohm and need to be at least a 1/2W type, if not bigger. Dropping a smaller voltage would require an even smaller resistance. Resistors of less than 1 Ohm are not standard products and are, therefore, quite expensive.
How about a couple of diodes to drop 1.4V instead of the resistor?