Hey Jake,
I believe you'll need two diodes in line to the bulb to control the current flow to the bulb. The diodes prevent the current flowing "backwards" to the other side.
The ones I've always used, and others mentioned as well on FFCARS, were Radio Shack 276-1661 50V / 6A Rectifier Diodes. Since there isn't a Radio Shack on every corner any more, need to find them elsewhere. But shouldn't be hard to find. Note also they are banded and must be installed in the proper direction. If the light is being supplied with 12V and the other side of the light is grounded, the diode band goes toward the light on the 12V side or away from the light on the grounded side.
This may help:
My427SC_zpsgsppynnp.jpg Photo by edwardb123 | Photobucket
or this:
2014-02-17RTschematic_zpsbdpysnn4.jpg Photo by edwardb123 | Photobucket
https://www.radioshack.com/products/...ant=5717598981
http://stores.radioshack.com/wa/taco...nue-south.html
Bruce