Looking at the changes for 1986 brake light wiring described in the Volvo New Car Features manual, the sensor is monitoring the third brake light circuit separately on a blue wire from sensor pin 5. The drawing indicates there's a separate current winding for the third eye, around the brake light failure reed switch.

So there are three currents which have to cancel the magnetic field around that switch, making the type of lamp in the third eye important. According to a later model's owner manual, all three lamps have 21W filaments, although the L and R lamps are dual (1157 type).
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Art Benstein near Baltimore