Get yourself what's called ground washers, these are washers with internal or external serrated edges. Where ever you locate a ground wire from your harness, be it on the firewall, kick panel, trunk , hell anywhere; unbolt it , brush/sand the eye terminal and grounding location. If you have more than one terminal at a location use these washers between each connection and at the bottom of the stack at the grounding location. These will help to bite into and keep a secure ground connection. Also apply a corrosion preventative to the connections and the surface. Your issue really sounds like a poor ground or deteriorated ground wire impeding current flow.
If you have a good DVOM you can locate your voltage drops across your circuits. By connecting in one circuit and turning the another on you may see an increase or decrease in range. if you ground your meter and use the pos meter lead to check your ground circuits it shouldnt reflect any voltage if it does you've found your problem. Best of luck.