I think you would need a seriously abnormal level of humidity to get any noticeable condensation problem inside. First thing to do is check there isn't a pool of water in the spare tire well as that is where it sits if you have a leak in the boot.
If you have a pool of water you want to clean it up as soon as possible otherwise it gets worse and travels into the passenger seating area under heavy braking. If it gets even worse it will migrate into the carpets, even to the front footwells.
I had this problem in mine a few years ago as there had been a knock by the back right light unit and there were still small gaps in the metal, including one at the top by the boot weather trim. I cleaned and dried it all up before sealing it with 'Liquid Metal' silicon sealant and have never had any problems since. Did have to dry out my rear seats sponge for several days and treat with anti-bacterial sprays as well.
You can also carry a bag of rice around in the back or silicon/desiccant sachets to absorb any moisture in the car.
Hope this helps.