I think there is a miscommunication about which red wire. The red wire that is coming from your voltage regulator/rectifier box is a DC feed that will go to a battery and as WarnerSteel said, to your tail light and horn, signals, etc...The yellow wire from the engine harness connects to the voltage regulator yellow wire as you appear to have it, but you need to "T" the red wire from your stock wire harness into the yellow wire. If you want to test the voltage, with the engine running, there should be AC voltage with testing clips on the middle of the yellow wire and ground. It may be less than 12V at Idle, but should get there with a little rev.The red wire on my stock z50 harness just runs to the headlight switch I don't see how connecting it to the yellow wire would be an issue. How else would I switch my lights?