Okay so looks like Garmin got smart--upon initial startup it has to download mandatory map data, which basically includes the InstaSearch SID file, gmaptz.img, gmapbmap.img, and some other non-map image stuff; both a GMA and UNL file get created as a result of this; there is no way to get around this part. Once I d/l all of that but did NOT d/l the main NA map file, again the app prompted me to d/l a main map--this tells me that it still won't work unless you have one main map installed. What I did then was exit out the app, manually installed a 2016.20 NA img file (I unlocked it using gimgunlock+mapsigpatch first); then when I restarted the app it still told me I needed to d/l the basemap even though it sorta recognized the existence of the 2016.20 map I manually copied over (it had a green checkmark meaning that it was *already* downloaded); upon re-downloading the basemap (again) and restarting, the app promptly deleted the 2016.20 map I had previously copied into the app.
Note--deleting the GMA and UNL files generated from the basemap d/l won't fool it since it would be no different from starting the app for the very first time since if you do an initial app start the GMA and UNL files don't even exist yet so the app already knows that you have no maps.
Update--apparently the app must create/generate some kind of checksum on the img file that is downloaded via the app; I renamed both the SID and NA IMG files to something completely different (I kept the SID and IMG file extensions though)--although the app still asked me if I wanted to update my maps, I was able to cancel out of it and view the original map I downloaded, meaning that it still recognized the original 2015.20 map that was downloaded via the app, even though I completely renamed the IMG and SID files.
Guess I'll have to keep playing around to see if I can find a way to fool it.
Last edited by tonytone; 4th November 2015 at 02:12 AM.