I would verify the grounds are good. I'm not criticizing your splicing technique(s), but it is a "seasoned" vehicle, with multiple owners and issues. (I don't liked the words "old and aged" too close to home). Do one thing at a time, check and verify. The symptoms don't seem to be a "voltage supply" problem, but more like there is too much current for the available ground(s) to handle. This "excess" current must find a way to ground, when it does, you get very interesting effects, dimly lit dome lights, dash lights even wayward brake lights, and rough engine idling. There are many posts with similar issues.