There is a flasher module on many cars that allows current to flow through a peice of "special metal" that gets hot as current flows through it. When it gets warm it expands. Once it hits a certain temp, it will expand enough so that it breaks the circuit. When a bulb goes out, it changes the amount of current going through the circuit and the expansion/heating rate changes as well.

Positive side of circuit o--------o "special" wire expands upwards
Negative side of circuit o----*