First off, look for any cracks in the plastic main cover, then the screw off cover for the fuse. I'd also look into the screws, make sure none of them are stripped, they only thread into plastic. Never hurts to use a little sealant on both side of the gasket too. Check the grommets for all of the cables and wires, rips or just that they weren't seated right.
Hell if your really brave, dunk the whole thing in fresh water like in the sink or tube. The escaping bubbles would tell you right away where your leak is. just make sure to dry everything after.