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The 123GT came along late in the model chain. Volvo needed to have a car which had some extra parts for homologation for competition. Actually too late as Volvo dropped their factory team soon after. Extra lights were fitted, along with the alternator to support them for rally use. For the GT they made up a small extra wiring harness for these lights as an add on to the standard one. A compromise but the simplest and cheapest way to do it. The "relay for signalling" is normally called the flasher relay and has been part of the Amazon kit since very early on. I had a '61 6v B16 122S and that had the flasher wired into the indicator unit as standard and that meant you could be very impolite and have it pulled back to give flashing headlights as you bombed down the outside lane of the freeway/motorway/whatever/whereever. For some reason the USA didn't like flashers.
If you have a true GT you should be able to find this add on loom. It has a block of flat connectors that should be close the bottom of the bulkhead slope just below the hood hinge.