We run a Garmin GPSMAP76C mounted inside the cabin of our 2120. We also have a Lowrance with external antenna. If the Lowrance reception is a 100, the Garmin is a 98. No question the external antenna is better, but in our experience it's not really necessary.
I was able to power up my 182C Garmin with internal antenna yesterday on my 2120.She locked right in on a bunch of satellites so I see no problem at all.
My only problem was that this was a new refurbished unit that Garmin sent me because of the screen tinting for sunlight viewiing on the first one started to spot.
The new unit fired right up,and my waypoints were there,but lost all my maps.A mere 2-3 hours on the phone with Garmin Techs and we'll see tom if the chip is reprogrammed correctly.At one point the Tech made me lose all my waypoints while trying to transer to the computer.Thank god 2 hours later they called back and found the backup file when they replaced the unit.I'm keeping my fingers crossed for tom,might get splashed in the afternoon if all goes well.