When a circuit has an open switch, current stops because the path is interrupted, but the power source still creates a potential difference. That means voltage can exist at various points in the circuit even though no current is flowing. You could measure the full supply voltage across the open gap or on the side still connected to the source, while other parts may sit at different potentials. So it’s possible for voltage to be present in at least part of the circuit with the switch open.

A rheostat is a device that varies resistance to control the current in a circuit. In cars, the instrument panel dimmer switch uses this principle to adjust dash-light brightness by increasing or decreasing the resistance in the lighting circuit, which changes how much current reaches the instrument lights. That makes the instrument panel dimmer switch the best example of a rheostat. Other items listed aren’t variable resistors: the ignition switch simply connects power in a selective way, a relay is an electromagnetic switch, and the brake light switch functions as a sensor that turns lights on when the pedal is pressed.

The key idea is that the fuse protects the circuit from overcurrent, so a motor that draws more current than the circuit is designed to handle will blow the fuse. If the new blower motor has lower winding resistance than the original, it pulls more current at the same supply voltage (Ohm’s law: current = voltage / resistance). That increased current can exceed the fuse’s rating and cause it to open. To check, compare the new motor’s winding resistance to the spec for the original motor; a significantly lower reading or a shorted winding confirms why the fuse trips. While a defective fuse, damaged wiring, or a stuck switch could cause problems, the most direct and common cause after replacing the motor is a lower resistance in the new motor leading to higher current draw and a blown fuse.

Open circuit voltage reflects how much charge is stored in a lead-acid battery when no load is on it and it’s given time to rest. For a typical 12V lead-acid battery, the voltage rises as the battery’s state of charge increases. At about 25% charge, the resting voltage is approximately 12.06 volts, making that value the best match for the question. If you see around 12.50 volts, you’re at a higher state of charge (roughly mid-to-high), not 25%. A reading near 11.80 volts indicates the battery is fairly discharged, while about 13.00 volts suggests the battery is being charged or has a surface charge from recent charging.