As I suspected it's a 'stock-standard' 3790 device with a custom Logo. Before I help you modify it, you need to make copies of 2 of its regions for safety. Download this file 'Garmin.rar':
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it directly to a clean empty microSD of at least 1GB size. Firstly ensure the card's format is either FAT16 or FAT32. With the device fully off, place the card in the nuvi and turn it on. If it has a low battery either fully charge it first or only do this with it connected to an adequite power source such as the vehicle charger. Wait for the procedure to fully finish indicated by the device rebooting. Remove the card and look at the contents, there will be 2 new files in the root named 16.bin and 41.bin. Keep a copy in a safe place of the 41.bin because that's your device's non-volatile region which may help to restore in the future. The file named '16.bin' is the one I need to see because that's its present logo region. Upload it somewhere and post the link please, preferably use Zippyshare or similar free site.