Water sloshing about - behind seats - clear blocked drains?

Oct 22, 2020
I bought a second hand Z3 in summer and now its getting wet I think a lot of water is getting into the car. Inside feels dry but the boot is quite wet - I thought it was just a leaking brake light (i am going to get a new gasket as this is defo leaking a bit) but Im wondering if rain water is either coming through a small gap at the back of the hood, or if the rain water is travelling down through the designed route but is stuck at a blocked drain point.

Can drain points be blocked? where are they?

How can i get into the between the boot and behind the seats because it sounds like a lot of water sloshing about in there!!