my switch is "2" "both" "1" "off" from left to right & it is a make before break once it is left of "off". The alternator and starter is only connected when the switch is other than "off".
Unless the switch is in the "both" position when starting, the GPS will go off because of the drop in voltage when starting the diesel. I do have a .5 v drop to the instrument from the instrument panel connection altho i am using 16 guage wire to the Garmin pigtail harness. I am thinking about soldering an other wire in parallel to all the panel connections.
I already mentioned in an other thread the finding that an other guy found by measurements that the battery switch in "both" does not cause a higher charged battery be drawen down by a lower charged battery unless there is a cell failure.