Then regulate it down to whatever the LED units need, or use multi-voltage LED drivers (some will work on anything from 12V to 28V), or use a simple constant current circuit to directly drive the LEDs.
EDIT: A quick look reveals plenty of options that are nominally 12 to 24V, not all of them for automotive use. For automotive use, they say 12V or 24V, but in reality the systems are designed for 14V or 28V as these are the actual system voltages with the engine running.
EDIT: If the LED unit is basic enough (just LEDs and a resistor), you can "convert" a 12V one to 15V by adding a series resistor of the right value - which would need to be calculated for the given LED unit.
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