I took apart a broken trailer light converter box (4 wire to 3 wire + GND) because I would like to make my own.
I drew out the circuit board and attached it...
There are two transistor type components one wasn't working but the other was so I was able to see it's operation.
The one on the right (the one that worked) received 12.6V from either the "L Turn Signal" or the "Brake" to pin 3 then if the base voltage on pin 1 was equal to 12.6 the Emitter (pin 2) would be off. But for example, if the L Turn signal is on, the voltage to pin 1 drops to around 10.5V (because the 12.6V will go through the 620 Ohm resistor then through the brake bulb to GND). This voltage drop on pin 1 turns the pin 2 Emitter on.
I think it's a pretty cool circuit...The component on the right was made by ST and labeled "W2M10S (or 5)" "TIP 127"
The component on the left was also made by ST and appears to have been labeled "W2J10S (or 5)" "TIP 127"
I can't find anything on either component on Google or ALLDATASHEET. Also I'm not sure why they were labeled differently when my testing of each side of the circuit seems to indicate they function the exact same (i.e. turn on with a voltage drop to the base).
Can anybody tell me what this is, and it would be great if anyone can suggest an equivalent part number as these parts seem to be obsolete.