Disclaimer: Use the following at your own risk; I am not responsible for breaking your system. I have come to realize though that this process mainly messes with the Map files, so if you screw up anything technically you can redo the process to fix it, it doesn"t mess with the underlying Garmin software so that still gives you access to fix issues. The links below are not mine, again use at your own risk.
I had created a thread in General Discussion but I think it is more fitting for it to be here. Anyways here are the cleaned up directions to update your maps in your car. Using this method you can get quarterly updates for a flat one time cost of buying a ~$220-280 GPS.
Tools required:
Garmin 3490LMT or 3590LMT GPS unit
gimgunlock
JetMouse
Garmin Express
8GB+ USB Stick
CTP2012 / 2012.5
1-Connect Garmin 3490LMT or 3590LMT to your PC
2-Run Garmin Express
3-Download CT NT 2014.20 NA update
4-Copy the following files that is downloaded to a USB stick
Garmin/ASR (folder+contents)
Garmin/SID (folder+contents)
Garmin/SQLite (folder+contents)
Garmin/GMAP3D.IMG
Garmin/gmapprom.img
Garmin/GMAPTZ.IMG
5-Create Garmin/JCV (folder)
6-Download: http://download.garmin.com/support/2.../D3060110A.JCV and put it in JCV folder
7-Put car in run mode (with engine on or not)
8-Turn all unnecessary features off (i.e. A/C)
9-Leave your radio on (needed to recognize a USB is plugged in)
10-Goto the Navigation screen
11-Plug USB stick into vehicle, it will start Validating the USB stick
12-When the update prompts, follow the menus, when it askes for an unlock code, use JetMouse
13-Let the system update (1-1.5hr process, if engine is not on, the car will shutdown half way through the process, you will need to put it back in run mode immediately, so ensure you stay with the car if you do not have the engine on)
14-It will either give a Maps updated successfully message or Reflash failed message. Just verify that the maps loaded fine, but you should be GTG.
CTP13
1-Connect Garmin 3490LMT or 3590LMT to your PC
2-Run Garmin Express
3-Download CT NT 2014.20 NA update
4-Copy the following files that is downloaded to a USB stick
Garmin/ASR (folder+contents)
Garmin/JCV (folder+contents)
Garmin/SID (folder+contents)
Garmin/SQLite (folder+contents)
Garmin/GMAP3D.IMG
Garmin/gmapprom.img
Garmin/GMAPTZ.IMG
5-Run gimgunlock on GMAP3D.IMG and GMAPPROM.img
6-Create text files GMAP3D.unl and GMAPPROM.unl and paste unlock code from JetMouse
7-Put car in run mode (with engine on or not)
8-Turn all unnecessary features off (i.e. A/C)
9-Leave your radio on (needed to recognize a USB is plugged in)
10-Goto the Navigation screen
11-Plug USB stick into vehicle, it will start Validating the USB stick
12-When the update prompts, follow the menus
13-Let the system update (1-1.5hr process, if engine is not on, the car will shutdown half way through the process, you will need to put it back in run mode immediately, so ensure you stay with the car if you do not have the engine on)
14-It will either give a Maps updated successfully message or Reflash failed message. Just