I have no idea how to fix this as I don't even know where the leak is coming from. It is very wet in the sill and I have to use a towel to soak it up. Rubbish at DIY. Don't have lots of money, but would pay some one to fix this.
The water could be coming in on the top of the window could be the seal is perished do you have a line of caps covering the screws may need doing up try a small turn to each screw
If it's a T25 & leaking around windows in rear side or rear of van then suspect rotted window edge where rubber sits. You will need to remove window which is easy & see what state it is in. If metal has rotted away you have 2 choices. Have it welded ie do job proberly or bodge it if you can with PU sealant.