I have a truck with a plurality of lamps, of 12V, 21W.
Sounds like the 1156 (ANSI number) that was popular in older US cars
and trucks. Usually available for about $1 apiece, cheaper if you
buy a box of 10 at NAPA. For a little more money you can get a "long
life" version with approximately twice the rated life.
They don't last long and replacing them is a tedious chore.
How's your electrical system? If you see much over 14.0 volts at the
battery, or over about 12.8 volts at the fixture, with the engine
running at 2000 rpm (gas/petrol) or, say, 1500 rpm (diesel), fix that
first and your lamps will last longer.
Were the fixtures installed by the manufacturer, or a later addition?
If added later, how well are they mounted? Loose and floppy fixtures
burn out lamps. Are they well sealed? Water leaking in will burn out
I want to reduce the voltage to each bulb to take the load off the
filament and so extend its life.
This also makes it dimmer, which makes your truck harder to see.
Dropping the voltage to 90% of rated voltage will make the lamps last
about three times as long... but it will also cut their light output by
There are LED replacements available, but they usually send all the
light in one direction, which is not a good match for the original
lamp. The fixture will be highly visible from one direction only and
hard to see from all other directions.
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.
Normally, if a voltage reduction is needed, it is done inline with the
wiring to the fixture.