Hmm, that's weird, it ought to work.

o The Curtis adapter red wire is indeed for brake lights and connects through to pin 6, the yellow-brown wire, on your Volvo "pre-wired towing hook" connector. You sourced a European standard Type 12N round 7-pin trailer connector with ISO 1274 pinouts as I recall, so we're talking the correct pin 6 for brakes on your Volvo. Note that on a North American 7-pin trailer connector, pin 6 is still for brakes (right brake/turn light), but is a green wire, not a red wire (red is on pin 4 for battery 12V to the trailer).

o As you'll recall, we determined from the Volvo wiring diagram that the yellow-brown wire is ahead of the bulb out sensor so won't cause warning light problems.

o I presume you weren't accidentally using the 4-way hazard lights for testing instead of depressing the brake pedal.

o I also presume you're not hooking up your test light to pin 2 (brown wire) on the flat trailer connector expecting brake lights. That's for the park lights and trailer marker lights. Brakes should light on both pin 3 (yellow wire, left brake light) and pin 4 (green wire, right brake light).

o I presume for your test light you've been using the white wire pin 1 on the flat trailer connector as ground, not another chassis ground. If not then double check you've attached the black wire to a good ground, either pin 3 in the Volvo connector or better still a closer chassis ground, such as the threaded stud the taillights use AND that the converter fuse is installed.

o If you've got a voltmeter, verify that pin 6 on the Volvo vehicle connector is providing 12V when the pedal is depressed.

o If you don't have a meter, run a temporary jumper wire from the red wire over to a taillight brake bulb to see if it now works.

o If all that checks out then there's always a chance you've got a faulty converter. Jumping 12 volts onto the red wire with the black wire connected to chassis ground and measuring voltage on both flat connector pin 2 or 3 would confirm that.

Good luck. You're almost there! Somewhere in the above should be your answer.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now