The dew detector light is much like the engine light on a car. It comes on for a multitude of reasons, not just moisture.
Sometimes it just needs to have a cleaner run through it for 10 or 15 seconds. I would try that first.
Buy a dry type head cleaning tape. Do not buy the type where you have to squirt liquid onto the tape.
Sony, Fuji and Panasonic all make a dry type cleaning tape.
Moisture can build up in the camcorder if it's been stored an an area that isn't dry enough or where temperatures are not constant. The culprit is usually the tape that has been stored with it. After a while, oxides start falling off the tape and clog up the works.
The camcorder actually has a miniature dryer built in.
It only works with the AC adaptor and not with the battery.
Remove the battery and connect the AC adaptor.
Turn the camcorder to camera mode.
Open the tape door.
Let it sit that way for 2 to 4 hours.
Then unplug the adaptor, put the battery back on and see if that changed anything.
You can re-set the camcorder by removing all its batteries and power sources. That includes the little flat, round battery that keeps the clock and calendar working.
If none of this works, it needs to be professionally cleaned.