Would a DPDT switch work to turn either one or both devices in this circuit?
simulate this circuit &ndash; Schematic created using CircuitLab
Figure 1. A circuit suitable for demonstration. A single-pole, double-throw (SPDT) switch is required.
In a real car the headlamps are typically 55 W on filament lamps and the circuit would have to switch more that 10 A into cold filaments. For this reason the switches control relays which do the hard work.

I misread the requirement for always on DIPs.



simulate this circuit
Figure 2. A single-pole, single-throw (SPST) switch will suffice.