How to add second, lower power source to car fog light?
simulate this circuit &ndash; Schematic created using CircuitLab

A basic circuit which ought to work would be as above.  The diodes should be rated at least 2A - the 1N5400 is 3A.  The tricky bit is going to be the resistors.  If you put 5W through a 21W lamp, it will glow feebly.  You would need some trial-and-error to find what actually works.  The resistors need to be high power wire-wound ones.