Most likely it needs a latch assembly. But you should look to confirm the switch is bad.

Sent from my C771 using Bimmer App

I know with my 2003 3.0i, I have to have all the doors closed then open the trunk lid. I have to let the tail gate go all the way down and slam it shut. Then close the top hatch firmly. If I leave a door open it will not stay secure. Crazy design but it has done it since i have owned it.

Try that before spending $$$$.