It's easy to unlock file-system of iPhone devices up to iPhone X - there a checkra1n exploit can do jailbreak and you can gain control over installing unsigned ipa-s and explore file-system. But... about Garmin maps - i don't know to exist a patched Garmin StreetPilot® for iPhone® [ it's official only for USA and Canada ] ... 
[Please Register or Login to download file]  about how to use old iPhone 4 for navigation unit can see here 
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