Honsetly, Just ran into this myself. I took apart the rear interior trim an paneling off the hatch. Took apart the motor, tried to see if my rear wiper motor was seized. No play at all on the shaft that connects to the wiper arm. What I did discover is that my grommet on the back window was letting in moisture as its misshaped now an letting in water. * no more drive threw washes for me*
ANYWAYS. The grommet is discontinued at the dealer, I managed to piece together some left over grommets that were rubber. You could find some in a dorman assortment display or local automotive store. Just to build up around the seal to prevent this again.
I live in Canada myself an this caliber is showing some signs of corrosion issues, so long story short. The wiper motor itself was seized the armature was rusted so bad it wasn't making proper contact with the rest of the motor. So placing it in a drill and spinning its with somewhat agressive sandpaper to bring back the metal finish helped the armature. The inside housing I used a dremmel styled bit to sand down the rust. A bit of brake clean an some lubrication from penetrant oil on some locations and she was back together and working. Bear in mind when you do take it apart, Make sure you pay attention to the White disk " and its golden contact points " and how they aling. This tells the motor when to turn on an in which direction. Also if this part is burnt or damaged this may be also where your problem lies.
I got mine back together, I somehow made it so it actually waits till you spray than wipes the rear window. Before it was wiping an missing the spray entirely. Almost became a game before to clean your rear wiper lol.
anyways hope any of this helps.