[SwissXC90]

Dry out the back
Remove the access panel to the boot floor.
Remove any stuff that's in there you can remove. You want as much visibility of where the water might come from as possible.
Sprinkle talcum powder over all the rubber seals and the metal work around the tailgate seal, over the rear cargo area so there is a white powder coating on everything.

Now you can see a water trail when it happens, and see where it comes from.

Close up the car.
Park the car in the rain, or hose down the car liberally.

Open the tailgate and look for the water trail.

Can be as simple as the boot rudder seal not sealing tight enough any more. Replace with new, OR do the hack of putting 1/4 inch plastic pipe inside the rubber seal air space to give it more volume.