Examine your stock bulb. Make sure that the labeling on the OEM bulb is the same as the new HID bulb.

For example, this particular vehicle uses D1S stock bulb. You'll need to make sure that the HID replacement bulb is also labeled as D1S.

D1S and D3S are not interchangeable, so if you find that the labeling does not match, you will need to exchange for the correct size.