Rubber stopper at end of drain pipes and returning mold

Hi i recently brought a 92 mk1 but ive had returning mold on the roof. Today i pulled up the back carpet to drain the internal drain pipes already it was damp. When putting a drain cleaner tool down the pipe it was getting stuck looked under the car the pipes seem to have a very hard rubber at the end which was wasnt letting water drain . I was wondering if anyone else has had this problem ive thought of drilling a bigger hole or cutting that part of the pipe out. Also just replaced the roof seal.

Thanks