If you hit "max" ac button the Aux Fan should turn on. If it does not check the wire connectors next to fan on the passeger side (should be easy to see/find). If the connecters are good then you probably have a faulty Aux fan.
Dealer replaces it for $550 (new fan) plus 4 hours labor and they will slap on
$200 for a freon evac and recharge.
A recent thread talks about the best price for Aux fan online. low $300s i think.
If you are handy, try replacing it yourself, and save about $1000.
When replacing by the "book", the front bumper needs to be removed completely. But my indy said that there is a way to do it by just loosening the bumper. the old fan slides out and is replaced in much less time/labor. (I don't know the details, maybe somebody else could chime in... )

I would guess that the freon level is not low since the ac cools as the car gains speed. (wind entering the kidney grills does the job of the aux fan). So if you do go to a dealer tell them to not perform the freon recharge (unless the fan can't be replaced without evacuating the freon. I'm not sure)

Good luck,
Rui

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