So you got it working then? Good deal, just sorry I was too late to give input.
The same company that makes halogen lightbars makes a 2-way alternating flasher unit that you just plug in 3 wires and you're done. Similar units were used way back when as well.
I have an old police/fire spotlight from Unity that has a spot on one side and a red flasher on the other that uses a standard two wire flasher. Guess it all depends on the company that did the original wiring as to how they ran the circuts

1956 GMC 370 dump " 'Tater "
1970 VW Volksrod "the Black Bomber"
2007 Chevy Avalanche
2020 Jeep Cherokee Trailhawk,
2005 Harley Davidson FLHTCUI

I dig all cars, old and new, whether they were hammered out of American iron, German steel, or Japanese tin cans. Being unable to appreciate them all is missing out on a world of great things.
But thats just MY opinion! :P