The licence plate lighting of a Bus 08/71-07/79 is screwed on the outside of the enginelid and to obtain a good seal against water and dust, it is sealed with a rubber. 
This rubber dries out in the course of the years and will tear irrevocably when dismantling. Replacement is the only solution. Like original, this rubber has 4 tabs so that it stays in place and does not come out and does not seal well.