Hello: The top of the heater is foamed into place. Removing it will almost certainly ruin it. So, the sheet metal top needs to stay in place, but you can dig out foam at each fitting and make sure it’s dry.
I would do this and leave it under water pressure. Then take a paper towel and get things dry. In a minute use a dry towel again to see if there is any “new” water. That’s a way to pinpoint a leak. 😉