I believe this is what you are asking about and what you seem to be missing. These photos are taken of my '74 CL360 so it is the exact same wiring harness you have.
Here is everything together...
Here is everything blown apart...
On my '74 CL360 parts bike, those wires are just connected together. I'm not sure if it works this way, but I don't see why it wouldn't work this way since my non parts bike CL360 has those 2 wires connected using a 2pin connector. There were some numbers, arrows and symbols embossed on the end of that black plug but I forgot to take a picture of it when it was apart. I would gladly take it back apart for another but that "white" 2pin connector it's going into is turning yellow and cracking so I would rather not mess with it anymore.
Also, I don't know how you were testing all your electrical stuff, but I noticed when I was testing mine, I needed both directionals for one side otherwise it would just stay on and not flash. Once the other directional was hooked up it would immediately start flashing. I think there was also something goofy with a light coming on in my gauges when the headlight wasn't hooked up too. Once all the lights were hooked up everything worked great. If you have other questions about the wiring let me know. I've got two bikes here to compare everything to (though the one bike hopefully won't be here much longer).
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Now, some questions for you... what exactly is that 500 tank off of? What is it's capacity? What side is the petcock on? How close do the front C tabs/hooks line up with the studs on the frame? Can you get a picture from the back of the bike with the tank on it so I can see how wide the tank is in relation to other parts? I kinda like the shape and I think it would be pretty easy to lengthen it for my CL360.
Thanks,
Mike