Had a rummage round t'internet on this and went down a rabbit hole on JDM satnavs. (Sure what else would you be doing..)
That unit is likely to have been made by either Pioneer or Kenwood, and on those that Full-W message is supposedly short for "Waypoint memory full". Clear that and you might be back in action. It could have an SD card in it somewhere, that could be full or just borked if it's old, be worth checking if it has one and change that. There's also supposedly a reset pinhole somewhere at the back as well if you can get it out, that might clear it down as well.
If you do get it going supposedly all the maps/menus are in Japanese so you'll have a bit of craic with that. Replacement units pretty much non existent ,looks like they were very rare.
All from various forums and whatnot so pinch of salt on that info.
If you've no joy wonder could you stick in a small android tablet instead of the oem unit? I did that with mine using an empty satnav cowl I got off ebay. It's not perfect but not bad either, handy for maps.
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