k7elp60 said:
I would assume that the tail light fixture is going to have three bulbs. There will be 4 wires to feed the fixture. 1 common, 1 left turn, 1 brake, and 1 for right turn. Is this right?
Assuming that the answer to this question is yes, then what you need are 2 Schottky diodes.
You will have to buy diodes that can withstand the current through the filament when it is cold.
So measure the resistance of the filament with an Ohmmeter and calculate the current that will flow initially. The current will reduce as the filament warms up, but you need to be sure that the diodes won't blow on the initial current.
I suggested Schottky diodes as they have less forward voltage drop than Silicon ones.
For example, the 1N5819 is rated at 1 Amp max continuous, but if you look at its data sheet, it will tell you what the allowable max peak current is.
However, I suspect (without looking at the data sheet) that the 1N5819 won't be able to withstand the current, so you will have to look for one that will.
Assuming that your bike has a negative earth, connect the anodes of the diodes to the brake wire and the cathode of one to the left side lamp and the cathode of the other to the right side lamp.
The diodes will prevent the centre brake lamp and right blinker from glowing when you turn on the left blinker, and vice versa if you turn on the right one.
But when you apply the brakes, both diodes will conduct and all three lamps will glow.