Bongo Fury

When I bought my bongo, the dealer fitted a rear fog light and switch. When I put the lights on, I could switch the fog light on and the switch would light up and go off when I switched the fog light off.
Now, when I switch the lights on, the fog light switch comes on, even though the fog light itself doesn't. The switch remians illuminated until I switch the lights off
Checked the bulb and that's ok
Any suggestions anyone

Check the wiring, sounds like the live wire's maybe chafed through & earthing? Or it's got wet inside the foglight?