So I have a 1990 ranger and the left rear brake light is out and the other works fine, but when I turn on the light they work but the left one will still won’t brake. I’ve swapped the bulbs and the fuse but it won’t brake.Anyone have any suggestions to fix this problem.

 

Welcome to the Forum

1990 uses what is called a Two Wire system
Brake and turn signal SHARE 1 wire to left rear and 1 wire to the right rear

And they share 1 filament in a dual filament bulb
So on the left side rear, for example, there will be one Dual bulb with 2 wires, its used for Park lights and brake/turn, also has a Ground wire
There is also a single bulb for Reverse light

So does the rear Left turn light flash?
If so then the wire to the back, and the bulb are OK

The problem is in the Multi-function switch, the turn signal stalk

When you press brake pedal down the brake pedal switch sends 12volts to the Turn Signal Stalk, not the brake light bulbs
The 12volts is then split to the Left and Right rear turn signal wires that go to the rear
And both brake/turn filaments light up

If only one side lights up for brakes, then turn on the turn signal for the side that doesn't work, if filament works, starts flashing, then the wire and bulb are OK
The problem is in the "Splitting of the 12volts" inside the Multi-function switch