Diode size for Fog lights

I am installing fog lights on my XJ, with an OR-FAB bumper. The fog lights can only come on with the low beams, in order to pass inspection. I would like to use a hidden switch to power the on/off switch. That way I can have them on with High, and Low for off road. One side of the hidden switch would be to the low beam wire under the dash, and the other side would be to the Headlight wire under the dash. That way only one wire has to go out to the relay. I know I need to install Diodes on both wires, but what size do I need to install. Any help would be nice.

Why do you need diodes?

I thought it would feed back. You are right since I am using another switch I don't need diodes. If I were using two different power sources, then I would need diodes. I'm getting too old for this.