You say "hid" in your description. The base of a HID bulb for the foglights is the same as the halogen bulb base. So if you did, in fact, install HID bulbs without a ballast, they caused the failure.
An electrical ballast
is a device intended to limit the amount of current
in an electric circuit
. A familiar and widely used example is the inductive ballast used in fluorescent lamps
, to limit the current through the tube, which would otherwise rise to destructive levels due to the tube's negative resistance
Ballasts vary in design complexity. They can be as simple as a series resistor
, or a combination thereof or as complex as electronic ballasts used with fluorescent lamps