If the outside tv has a headphone jack, You have couple of options One above or.
1. Run a 3 conductor shielded cable from the headphone jack to the aux in on the stereo.
2.Get a bluetooth transmitter and pair it with the sound system. Connect the bluetooth transmitter to the tv via a 1/8 stereo plug. This solution is much simpler, but depending on the BT transmitter design, could result in a delay between the actual audio and the video. This means sound will be out of sync with lips. Not an issue if watching sports, but can be annoying for movies.
2015 Freedom Express 248RBS
TV 2015 Silverado HD2500 Duramax
TST Tire Monitors
Honda 2000I + Companion
2 100W solar panels