Attention!!!
When you connect a "custom KMZ map" compatible device to Basecamp and you use a kmz map from your device and change its "Draw Order" (or ...opacity) in Basecamp, then this "new" Draw Order is automatically updated in your device (is written in the doc.kml file inside your kmz file).
Garmin says the kmz's Draw Order to be 0 (zero) so that the kmz map is drawn under all other routable map elements.
So use the same kmz file but in "My Collection" instead of your device, or before you close BC and disconnect your device, check that the device's kmz Draw Order is again set to zero (bottom).