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There are two types of sensors.
The older style 97-00 has a separate o-ring.
The newer style 01-04 has the o-ring integrated with the sensor.
So there is no way to install the o-ring in the wrong place.
But just for the record it goes between the sensor and the inside of the rim.
There is a rubber washer that can be changed if yours is chewed up. It goes on the valve stem inside the wheel, a good idea is to put a light coat of silcone grease on the washer to help it seal.