We have a couple of options to set your Grand Cherokee WK up with a roof basket but you're going to need to add a pair of crossbars to your roof rails. There isn't any way to mount a basket directly to the side rails.
If your Jeep has the black and chrome track flush mounted rails that run front to back you will want the Rhino-Rack RVP Roof Rack for Fixed Mounting Points item # RRRVP27.
If your Jeep has the matte black flush mounted rails that run front to back you will need the roof... view full answer...