Small bulbs always burning out.

I have a truck with a plurality of lamps, of 12V,
21W.

They don't last long and replacing them is a
tedious chore.

I want to reduce the voltage to each bulb to take
the load off the filament and so extend its life.

They are bayonet fittings, and so I seek a small
thin disk of some resistive material to put into
the lamp socket to contact the center terminal, so
that when the bulb is inserted this will act as a
resistor so reducing the voltage by a small
amount. Clearly the inserted bulb...

Small bulbs always burning out.