Here's the best deal - by far:
This sensor IS an EXACT fit/replacement for your downstream sensor on a 1998 Toyota Sienna --- down to the wire holding clip.
The downstream sensor's connector on a 1998 Sienna is located under the front passenger seat.
*Remove the 4 - 14mm bolts holding down the seat.
*Prop the seat upward.
*Pull back on the carpeting slit to reveal the connector.
*Pinch the connector to remove the female end of it.
*Use a needle nose pliers to pull up and remove the rubber grommet from the floorboard, then push it down through the hole.
*Jack up the right front (behind right front wheel), then use a jack stand for safety.
Note: Make sure the exhaust pipe is well-warmed to the touch - for easier removal of the old sensor.
*Use a 22mm box wrench (counter-clockwise) to remove the old downstream sensor from the exhaust pipe.
*Install the new sensor (already contains anti-seize compound). Do not overtighten.
*Snake the wires/connector up through the floorboard.
*Use your needle nose pliers to pull the grommet up through the floorboard, then massage it into place with your fingers.
*Click the female connector into place.
*Re-install the seat.
Hey! You just saved big $bucks$ by doing this yourself.