As noted, make sure everything is connected as it should be. The rain sensor is in a black box near where the rear view mirror is attached to the windshield. When it's dark and the wipers are in sensing mode, you should be able to see a faint glow when you look at the sensor from outside the car (you may have to get close to see it). If you don't see the glow, chances are the wiring didn't get reconnected correctly or something got damaged in the change-over.
There should be a "cut-out" in the factory tinting that matches the sensor housing--the glass should be clear where it's attached (as I recall from my '04, it was a very distinct clear area). If there's any tinting/patterning to the glass over the sensor, it's either that the sensor's in the wrong position or they installed an incorrect windshield (one for an LS without the rain sensing wipers).
If the glass is clear and you can see the faint glow, try cleaning the glass over the sensor--it's important that it not be contaminated with Rain-X, wax, or anything else that will interfere with the sensor (though it's certainly safe to use them on the rest of the windshield). Bon Ami is good for this; if you can't find it, just get a good kitchen cleaner that's safe for use on glass surfaces (someone recommended Barkeeper's Friend, but I haven't tried it).
