mine is on the outboard side of the driver's side front wheel well...toward the rear of the well.
They are very rugged, so don't worry about the tires kicking up stuff and pelting the transponder. It must be no more than 2 feet off the ground and have a clear, unobstructed view of the ground.
They say that they can work through plastic, but the signal is weak when you do this and timing and scoring will send the chief steward to talk with you.
My wife was a major player in bringing transponders to the Southwest Division.
-- Colin Mason
ITS #07
Houston Region SCCA