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Dear Tom F.,
Good a.m. and may this find you well. I believe the bulb failure sensor depends on the difference in amperage "draw" between pairs of bulbs. Two working bulbs -made by different firms, so with different power "draws" - can "fool" the sensor, and trigger a "bulb failure" warning.
On a US/Canada 1993 940, the third brake light is powered via a Yellow/Black wire from position 5 on the bulb failure sensor via position 18 on Connector C1. On European cars, the third light is powered by a yellow-red wire, that is "jumpered" from position 3 on Connector C1 to position 18 on Connector C1.
The Yellow-Red wire powers the right brake light.
I mention this, because it shows that an uneven loading - the wire to the right brake light supplies the power to the third brake light - does not trigger a "bulb failure" warning. There may be a resistor inside the sensor, that accounts for this uneven loading.
To draw power for the spoiler brake light, you could:
(a) jumper a wire from the position 4 on Connector C1 (from which a yellow wire runs to the left brake light)
(b) jumper a wire from the left brake light.
I'd try (b) first, because it is a shorter "run". If it triggers a "bulb" failure warning, then I'd use option (a).
Connector C1 appears to be located under the dashboard, on the driver's side (USA/Canada). Position 3 has a Yellow/Red Wire and Position 4 has a Yellow Wire.
Hope this helps to "light up your life".
With every warm, best wish for Happy Holidays and a Good, Healthy New Year, I remain,
Yours faithfully,
spook
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