I've been getting an intermittent bulb failure light on the instrument panel when the brake pedal is depressed. This indication is now a constant (with brake application). I determined that the left brake light was, in fact, extinguished when the warning bulb was illuminated, and vice versa. I've had the brake/rear light bulb holder out, and checked the contacts with the light assembly; I've replaced the bulb with an bulb identical to the previous (although I'm aware of the necessity of having identical (Volvo) bulbs wherever there's a comparator in the circuit). I can see from the Green Book schematic that the power to all three brake bulbs is common coming in to the "rear warning sensor" block, and since the other two brake lights illuminate properly, the problem,I believe, exists from the junction through the rest of the sensor on to the bulb. My question is: has anyone out there opened the bulb failure warning sensor - rear (without destroying it), and if so, is there anything inside that might be replaceable. The diagram shows an oblong component in the circuit to the connector leading to the socket wiring , but no further info as to the nature of this bit.
I've found that this sensor can be quite expensive to replace, and there's a dearth of pick'n'pulls down here in the Vancouver (BC) area, so if there's any chance that the sensor is at fault, and can be repaired, I'd like to have that option open, after checking a few other possible faults in the system. Any hints out there?? Thanks.

Garth.