We should be able to use the simpler "SD method" to flash region 16 anyway, then there will be no need to use Updater.exe to flash an RGN file in preboot.
First let's check some things however. An aera device's settings allows you to choose from a selection of images to display during the second phase of boot. It will display the usual Garmin logo first and then switch to a custom image when usually it displays the Garmin logo both times. The second image can be changed using its tools menu. In Automotive Mode, on "Where To?/View Map" screen press Tools then Picture Viewer to see if the second image is shown in the selection. If it is then select it and press the "I" (information) symbol then uncheck the box next to "Display at power on?". That isn't the case in Aviation Mode. No splash image is displayed during boot unless the aera's in Automotive mode.
If that's not the problem and someone has loaded a custom image to display during boot in Auto mode then we can change it back in region 16. I'm happy to help you with that should you need it but first please report back confirming you have it in Automotive mode and there hasn't been an image loaded via Picture Viewer.