Had a look and the FSM shows only one fog light sense for the BCM, and only one wire leaving the switch for the fog lights. It must be another resistor multiplexed switch sense looking for different values for front, or front and rear fogs. If you wanted to have auto headlights and rear fog you'd have add a switch somewhere to control either the fogs or the auto headlight feature.
The easiest way I think to do it, both options would do a similar thing, switching the circuit to the BCM between the headlamp switch and a preset resistance to control the fogs or auto HL. Do you have somewhere you could add a switch? You'd need to run two or three wire to the switch to get it set up.
We know what resistance the AHL feature requires but if you were to swap your current switch and add a seperate switch for the fogs we'd need to know what resistance is at the switch between the fog output and whichever pin the input is from.
