I have a 93 mercury capri that leaks when it rains a lot; theres mold in it now... what do i do?

I've had the car for over a year now, and it's great when it's sunny out. When it rains though.. that's a different story. It leaks a little bit and now has mold behind the penger seat on the floor, and most recently there is mold on the penger seat because i left for vacation for two weeks and left a towel sitting there. (when it rains people don't want to sit on water... so the towels just usually stay there) I want to get it all cleaned and try to sell it...... What do I do????