Authentification failure seems to be a kind of protection for garmin-nav.app. I have purchased app and it works with any map.
Possible solution is to ban gold.garmin.com and bronze.garmin.com to 127.0.0.1 in hosts file (as _http://gold.garmin.com/OBN/OBNServlet and _http://bronze.garmin.com/OBN/OBNServlet are used for requests).
You may find initial problem research here:
[Only registered and activated users can see links. ]
Also I found that TargetConfig.plist has the option that can be changed:
has acceptable value:<key>navigation_map_style</key>
- top bar became blue<key>navigation_map_style</key>
- speed limit sign is "european"
If I change client_type_id in this file, application writes Authentification failure message.
Also we could expect multiple onboard applications: