If you make sure to get RED LED bulbs for the Stop/Tail lights (or rear Fog) then none of the light power is wasted in the red lens, unlike a white bulb where most of it is lost. This is worthwhile.
I’ve fitted a T20 RED LED (no longer available) as the Fog bulb and it is about three times brighter seen from outside the car than a T20 WHITE LED of same lumen (lm = brightness rating) and current usage from the same manufacturer (Aglint) in the same socket.
With the original filament bulb it does not matter which end is + or -.
Looking at this spare T20 W21/5W bulb it is wired as Pins 1, 4 = Stop, Pins 2, 3 = Tail. Also pins 1, 3 are underneath the wedge (in this photo) while pins 2, 4 are on top.
In the car only three wires leave the socket so I expect (say) pins 1 and 2 share 0V, and pins 3 and 4 go to Tail and Stop, typically 35 lm and 440 lm of white dropping to about a fifth of this after the Red lens.
But be careful, because while there is an agreed standard, some bulbs might (anecdotal on the web) connect the four pins in a different way to the contacts in the socket in our car.
With some of the LED bulbs polarity matters, others don’t care. If they say 12V/24V then they are much less likely to care about polarity and will have cunning control chips monitoring temperature and current.
Some LED bulbs include extra load resistors to fool CANBUS, these are irrelevant in an NC and a waste of power merely shortening the bulb life by cooking it needlessly.
I’ve been looking for a suitable Stop/Tail candidate since the middle of last year, but have yet to make my mind up, although I have replaced the weedy T15 reversing light (why did Mazda not fit a meaty T20 like the Fog?) with a compromise (gets much too hot) WHITE LED.
My criteria for the Stop/Tail (just me being picky)
- The Tail need only produce the same light level as before, thus allowing much lower power = cooler, so hopefully the bulb can stay below 55C (ish) in the enclosed space and then it might last for the claimed lifetime.
- The Stop should be brighter than before, and higher power is OK because it is intermittent duty, and with time to cool off, one hopes.
- Colour should be the same RED as the lenses on the car for maximum efficiency.
I’m tempted by some Phirips LEDs, but they are expensive for an experiment and the specification is not that great for the lumens (20/100 Red) although better than the Osram offering. The Tail will not cook the bulb, but is there enough welly from the Stop?
And of course these are not legal for use on the public highway if not original equipment.