Best place to spray, if you can see it, is the max air intake inside the car. IF it can be seen, it'll be from under the dash on the passenger side (might have to remove a cover panel above the passenger's feet), or behind the glove box.
You'd put the A/C on Max or Recirculate and spray into that area....you'd basically be spraying right at the blower fan cage, and it'd would direct the air right at the evaporator, which is where mildew accumulates.
You can also do it with the A/C on normal or Vent, from outside the car at the cowl panel at the bottom of the windshield, as suggested earler. The air intake on some cars is easier than others to see/hit with spray there than others. Some are covered by the cowl panel, which would prevent you spraying directly into the intake.