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How old is the GPSMap 188 in question?
Such spots may appear on almost any display given 10 years of exposure more or less... or perhaps fewer years in some cases. That's why I ask how old the display unit is.
There is probably nothing you can do yourself about the problem though. It isn't anything easy-to-fix, that seems quite certain. It will have to come down to a decision between repair-servicing the device (which, unless still new enough to be within the warranty period, would be at your expense), or replacing it. If the device is fairly old, of course, some would say, replace it because something else is likely to go bad by and by in a device that old.