Turning on an automotive relay when enough voltage is avaialble
Here is one approach (this assumes the bulb has one side grounded, if not then just flip everything over and use a P-channel MOSFET). 

The idea is that the PWM signal to the bulb discharges the capacitor C1 (through Lamp1 and D1) during the 'off' part of the PWM cycle until the lamp is almost completely on. 







simulate this circuit &ndash; Schematic created using CircuitLab