Passenger side brake light on constantly

I woke this morning to find the passenger side brake light on constantly, even with the ignition off.  The driver side brake light works perfectly normally, so it's not the brake light switch.  Any ideas?  It's a '94 2WD Hi-Ace.

Thanks in advance for any advice.

First thing I would do is look for a wiring fault at the light cluster.
Does the driver's side light work OK?

I would expect that if it's only affecting one side then the problem will be a short or bad ground in the cluster itself or the wiring specific to it...

Cheers,
W.

Do you have a trailer socket fitted? If so check for contamination/shorts in the back of the socket.

Turned out to be a broken spacer on the brake pedal arm, that sits between the arm and the switch that causes the brake light to come on, there was a red herring in that it was only one brake light as other one had popped over night whilst burning away.