Your meter goes between the two connectors. Use a wire between the connectors for the other leg. The factory fog is the load.
As I said earlier, " Won't tell you the max available, but may raise the bar over the SS3. "
The DD SS3 pulls 3A (38.5W), and works in that spot.
You could use a meter to check the draw on the factory light. Won't tell you the max available, but may raise the bar over the SS3.
If a manual with 4.7s can turn a 35" tire, it will be even easier for an auto with 4.46s. Auto has a lower 1st gear. Couple that with V6 and there is even less reason to stress about a gear change.
4.7 x 4.283 = 20.1301
4.46 x 4.714 = 21.0244