The mechanical part of the switch on most of these precludes turning on the fog lights by themselves. The only way I can see that you would be able to operate the fog lights independent of the parking/headlights, would be to install a bypass switch specifically for the purpose.

On my '82 300D, that would mean connecting a single-pole switch connecting the NSE and 58N terminals on the back of the light switch. A gray/yellow/green wire goes to terminal NSE and two gray/green wires go to terminal 58N. (I wouldn't bring in a new hot wire specifically for the purpose unless you isolated the fog lamp circuitry from the rest of the lighting circuits.) It might also prove beneficial to wire in an LED that would let you know when you had the fog lights on.

Cheers,
Wes