Before considering the switch, determine which lights you are interested to implement.
Remember, Driving lights compliment the high beams and should, by some states law, require that the driving lights are only activated when the high beams are on. When the high beams are turned off, so too should the driving lights be deactivated. In my implementation, the switch will not activate the lights unless the high beam floor switch is activated and will turn the high beams and driving lights off when the floor switch is flipped to low beams.
If, as you have said, you are going to implement FOG lights, these can be activated independently and need not be complimented by the head lights. However, when used with the head lights, low beams are the compliment as high beams would provide too much reflection and diminish, if not eliminate, their benefit.
So, which is it, Driving lights or Fog lights?
Either way, I agree with Keoke, and strongly suggest you provide power to the lights through a relay and only add low-amp switching power through your original harness. Either way, tying other lighting functions would be beneficial, if not necessary, and determine how, after turning on the switch, how the wiring should be routed to switch on/off the relay.
One last point. When implementing my driving lights in the early '60s, I took a simple light switch and installed it out of sight on the bottom left side of my dash. When redoing my Healey in the late 1980, I decided that my use of a windshield washer was approaching ridiculous and installed a push button switch inside the original housing. This approach did eliminate the (never used) washer but provided a useful incognito switch for my driving lights without changing or adding to the look of the original dash.
Just my thoughts,
Ray(64BJ8P1)