Electronics/Electricity:
1.0 We have three choices for circuits, series, parallel and a combo of the two (series-parallel)
2.0 In a series circuit, every thing is one thing connected after another.
3.0 In a parallel circuit, things are connected side by side.
4.0 Much of what we are talking about is about series circuits. That is, for example:
(a) power going from the battery positive terminal to a fuse;
(b) power out of the fuse going into a switch of some kind;
(c) power going from the switch to a load (e.g., light, wipers, heater motor); and,
(d) power going through that load item and on to ground via direct connection to the battery or via a bolt or screw to something that is tied to the battery, to complete the circuit.
5.0 The total voltage to a circuit, be it a dome light, heater blower, or other electrical item, is dumped across the bulb or other item, UNLESS there is another source of resistance to the flow of current in the circuit.
5.1 If another source resistance is in the circuit, some of that voltage and total circuit power will be dumped the other resistance too. So each resistance has voltage drop across it.
5.2.1 If the ground is oxidizing (rusting), but has not completely oxidized , it will act like another resistance. Accordingly, there will be voltage and power dropped across the ground.
5.2.2 As already noted by Rick and others, you can check for a voltage drop across a ground. To do so, put the positive DC lead of a volt meter on the output (ground side) of the bulb, or other normal load in the circuit, and the negative lead to a known good ground, or the negative terminal of the battery. Any voltage indicates the ground is acting like a resistor, to a greater or lessor degree.
6.0 Adding other resistors to a circuit can produce tail chasing, head scratching symptoms. Just ask an experience square body owner, or someone who drives something with a European approach to auto wiring and wait for the sigh). This is especially true if there might be another, another path of least resistance back to the battery.
7.0 Since I'm babbling about tricks of troubleshooting, the "remove the battery ground and put a 12 volt test light between it and the negative terminal" trick should be mentioned. Since it fits in with the above, but from another angle and for a different problem.
7.1 If your battery is going dead for unknown reasons, you can use the aforementioned connection to determine if something is draining the battery.
7.2 If the light is on, some circuit is complete and draining the battery. However, keep in mind, the light could be very dim.
7.3 If you want to verify the test works, and the light appears to be off, just turn on the park lights and you'll see the test light come on.
7.4 With the park lights, ignition and other things off, and if the light is on, the radio may have an active circuit for the clock. You can pull fuses to remove the radio from the circuit. Then, if the light is still on, temporarily remove other fuses to see if removing one of them shuts the light off.
7.5 If pulling fuses did not shut off the light, someone may have wired something else that bypasses them.
For example, the former owner of my rig, a/k/a an electrical expert, wired my choke straight to the battery. Disconnecting it shut the light off (the choke should have been powered off my oil sender unit, so it was only receiving power when the ignition was in the run position).
