2004 X3 trunk leak?
 

So I used the windshield wipers and sprayed some water (used the steering controls) from the car onto both the front and rear windshields and let the wipers do their thing. Oddly enough, water started leaking into the trunk as I was spraying! Well not water, but the windshield wiper cleaning solution. I have yet to pour water on to the trunk and check if its a broken seal or a broken pipe, but has anyone had this problem? Obviously if tomorrow when I pour water on the trunk and nothing happens, it's a broken pipe. Is this an easy fix? When I spray water from the car, water leaks into the left side, top of the windows and seeps into the trunk.

Likely a blockage or leak from the rear windshield washer line, since that's where your leak is occuring.

Just drove through heavy rain and no water came through, which means it is the rear windshield washer line. Any ideas on how to fix this? Would love to try myself before I go to dealer

Try taking off some of the panels in the trunk area to see if you can locate the piping. In the meantime, don't use your rear washer.

Hopefully you'll find it and are able to fix it yourself.