I will look up gmaptool, thx.
After unchecking them one at a time, rebooting garmin, one appears to have the detailed data, and the other appears to be some basemap. This second does not show the map "squares" like the detail one.

Having just returned from baja, this would have been a handy tool - I tried the gps with its current stock map a couple of times, but the existing roads shown south of the border were very simple and most of the time I was shown driving offroad from mex 1...

Thx again for the fast response!