Max46, I'm not sure what you plan to investigate. In an automotive electrical circuit, virtually everything is connected in parallel. That means everything is getting 12 volts (when switches are on), everything is connected to a common ground, and there should be no interaction among loads. In reality, everything is connected together in parallel from the battery, and the circuits just have different fuses. I don't see how mixing bulbs can cause anything like you say you observed if the wiring is correct. If you find anything, I'd like to see it.