Could be a finicky bulb out sensor. It's a red can, about the size of a tomato paste can, sits in the relay tray behind the ashtray. The sensor measures the draw from the lights when you turn them on, and if there's is a difference, it turns on the warning light. Because it's in the circuit (fused power to brake switch to sensor to bulb), it also can be the cause of a non operating light.

The sensor has been known to give a false "light out" warning when bulbs of different ages and from different manufacturers are used on opposite sides. I usually replace both left and right bulbs when a bulb burns out.

I've got a finicky sensor in my 240 right now, and find that an tap with the handle end of a screqdriver sets it straight for a while. I'll probably end up replacing it - costs about $60 from FCP Groton.