okay this will require a little work, but what the cause is there is a warm air leak into the discharge side and water is building up and the drain hole is plugged. I have the same problem, just haven't got around fixing it.
What you have to do is first remove the outside cover. Next look along the front and sides for weep holes that might be blocked. Some of the newer ones have a drain line built in. It's full of dirt. You may have to unseal the condenser cover to clean the holes, if so, then you will need aluminum tape to reseal the joints.
Right now it's to hot to work on it during the day. Waiting for cooler weather. Good luck
Renegade (Jim) & Wrangler (Doreen)
& 4 attack cats, TJ, Shadow, Chessie, RJ
along with 2 who adopted us, Tom & puddy.