You may have more then one problem as you can see a wet spot on the top where I indicated. You need to remove the heat shield to see if it's leaking from one of the sensors in the manifold.
The most common reason to see a leak on the outside of the heater in that area is the pressure switch has gone bad. Remove the panels and you will se it on top of the manifold. Also, the temp sensor is in that area. If not there, then look at the hi-limits on the right of the manifold.
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