From my experiance, I would not wait till it seizes. The piece that the wiper pivot shaft and motor mount to is cast aluminum. I would be concerned that if the shaft seizes completely, you will not be able to remove the sahft without damaging some thing. I do not know of a clutch inside the motor. The motor is so small that I would doubt it.

Yes, you will need a tailgate repair kit (your wagon will need one eventually).

The only part I purchased was the black plastic cover (which covers the piviot shaft) which protrudes throught the back glass.

One important caution. The pivot shaft mechanisim is fastened to the wiper motor. The connection is splined and held on with a bolt. Mark the position of arm relative the motor mount with a pencil mark before you remove the bolt. If you do not re-assemble in the same position, the wiper will park (rest) in a diferent position on the rear glass. So you should test the wiper operation before reinstalling the trim.

Good Luck.