So I downloaded this gmapsupp.img file. If I want to use it in my Garmin 60CSx by itself, I can just transfer this file into the Garmin folder on the memory card and I will see the trails. I did this btw and it works fine. I can see the red trail segments. Of course, on a 60CSx, the basemap is pretty sparse, so there is usually nothing but the trail segments visible.
So how do I use it with the City Nav NA v2010.2 map that is already installed and also named gmapsupp.img? Is there a naming convention for when there are more than one?