Hi thank you for all that information.
Unfortunately this problem is the display driver IC that has failed. It s a cheap part but it is surface mount soldered tot eh main board inside.
Your Garmin could be repaired but with shipping and the labor it would cost more than a new Garmin, that would have more features and cost less that your original one did years ago.
I would recommend just replacing this one for a number of reasons , it is also what I would do myself lol.
I wish I had better news for you I really do.
In any case, I will stick with you until this is complete.
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Thank you and have a great day!