As above its a case of eliminating potential leak points. After checking the sun roof the next thing I would do is put dry newspaper or cardboard inside the boot and with it closed use a hose to spray water on the top of the bootlid only. Let the water run off and open the boot to check if there is wet inside. If there is none close it again and now spray water at the rear lights. Check it again.If the paper is still dry then it eliminates boot seal and light seals. Next potential leak point could be if you have any boot bottom grommets missing or not pushed in properly it will allow water to get inside the boot from underneath.Good luck.