After talking to my brother, I think you may not even have to go under the dash to re-wire the overdrive relay to work with the momentary switch that is built into the stock turn signal switch by using the existing purple/brown wire where it emerges inside the engine compartment.

I think that wire is tucked back into the main harness near the existing overdrive relay and you may be able to unplug a wire from the overdrive switch and plug in the purple/brown wire in its place. At worst, you may have to extend the purple/brown wire to reach the existing overdrive relay.

You have to unplug the smaller black wire that connects to the pole labeled 31b on the relay. There are 2 other slightly thicker black wires, but those 2 wires plug onto a shared pole on the relay and supply the power to the overdrive solenoid and the overdrive light on the dash.

Let me know how it goes.
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Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502