MR. Yonut;

I have trouble following your description of voltage drop testing...simpler would be to check voltage across Battery (Source), then at Headlight (or whatever load is being tested) with it's controlling switch(es) ON.

RPM varies system buss voltage because Alt output is variably contributing, so you are seeing voltage differences, but these are a secondary effect, not cause. Voltage drop has nothing directly to do with Idle RPM. Voltage Drops are a function of in-line resistance, which is RPM independent, so if you are reading 2V of drop anywhere, you will see it at any RPM and it's too much and should be corrected.

I understand about giving options, and differing opinions, and when I give mine specifically for the Volvo, it harness, and components, I support them with good science, and practical experience also from the Volvo, not other modern cars, where relays are typical for the reasons I have also given (and discounted as invalid for the Volvo).

Cheers