Agree with the others, leaks into the boot are usually down to a hardened aerial grommit or the high level brake light seal failing, both will allow water into the boot. Condensation on the boot lid inside surface can crop up, depends on where the car is parked and if the interior is already damp. Replacement brake light gasket from BMW cheap way to go but I did have a look on Realoem parts site, only shows the brake light unit with gasket at extortionate money. You will have to ask them if they can supply just the gasket. If you need a replacement aerial grommet get that from BMW, the ebay ones are higher price. Also need to get the correct one for your car as there are two types, one for early cars and another for wide body later cars.
No good fitting interior trim to the boot if the brake light is the cause of a leak you must fix that before covering the lid with a trim card otherwise its just going to get wet and soggy over time.
The pictures in this thread are what I did to my Z3 and Z4, used thin hardboard and covered it with thin foam followed by grey van lining material. All stuck down using an aerosol glue contact adhesive. The Z3 trim cover was fitted to the boot lid using plastic press in trim fasteners holes around the boot lid were used to instrall the fateners. The Z4 had to be fixed in using shortened self tapping screws so they didn't contact the outer boot skin and cause dents it was just a matter of drilling through the trim panel at suitable locations around the boot.
What I found on Realoem site here
BMW parts Z3 E36 Z3 2.0 Third stoplamp
www.realoem.com