The most common cause i have seen for water accumulating inside the trunk , is the outer tail lense gasket

these gasket are made of a foamy rubber material, over the years , they get hard and crack , when that happens , after a good downpour you might find water in the trunk..

replacing these gaskets is real simple

parts needed :2 gaskets, honda-bond or rtv silicone

90-93 accords,,part # 33503-sm4-003

94-97 accords,,part # 33503-sv4-ao1

you will need two ,they are the same for either side,,

tools needed : 1/4 inch 8 mm deep socket, small extension, rachet, i would not use anything bigger than 1/4 inch , a bit too much force and you might snap the stud in the lense

remove the panel covers, Disconnect the connectors that are plugged into the tail lense, remove the 8 mm nuts, remove tail lense,

Clean the area real good where the gasket will meet the body of the car, apply some honda-bond ( silicone ) where the gasket will meet the body, reinstall tail lense assembly..

DO NOT overtighten the 8 mm nuts as they will snap easily..plug the connectors back and you are done,

Always chk your work, make sure all the lites back there work properly..