After re-reading my post I realized I named the sensor incorrectly.
The BULB Failure Sensor is what it is called.

From the 700/900 FAQs:
Brake Light Failure:
[Query] The brake lights on this car don't work. The fuse is okay, there is power at the brake pedal switch and the switch works. I get no continuity, power at the white junction block leading to the rear harness. Could it be the bulb failure relay itself or is there a little rats nest in there somewhere. [Response: John Sargent] The brake light failure in our 87 745T was caused by the bulb failure relay. You may be able to re-solder, or you can replace it. The bulb failure relay is in the front left corner of the relay plate, and is red and round. [Self-Diagnosis from Guy Jett] It was the bulb out relay. The counterperson at the local Volvo dealer said "Oh yeah, they go out all the time." Note on diagnostics. It was helpful to disconnect other bulbs and see that the bulb out relay still was not working. I think I will keep a bad bulb or two in the future for such testing. [Jason Kneier] I opened the bulb failure relay up and traced the path the brake light circuits took. I saw two suspect solder joints, both of them 'bridging pins' that connected between the three boards, and so I resoldered them and put the relay back in - Success!