Every map (.img) has its own FID, Maps threads show this FID.
With this FID and your unit ID you can create the .unl file as explained in linked thread.
In few words, build your usb pen with the mentioned structure (see that thread) get your gps unit ID, build the .unl file made for it and map FID with JM kg. Insert. Usb stick in your unit and update it.
I don't know your navigation unit, but reading threads helps.
I think you can get unit ID even if navigation doesn't work. I think unit can be started and you can reach some options to get it.
You must read!


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