Diode selection for truck wiring light application
You need something that can handle 5A at least, if it's a 55W bulb!
Also, there will be about a volt of drop across those diodes.
Also, you need at least 50V rating to handle load dumps when the starter is involved!

Digi-key seems to indicate 10A1CT is the cheapest through-hole that matches the specs: http://www.digikey.com/product-detail/en/10A01-T/10A01CT-ND/1922678