In my GLX+ touch screen radio it has an input selection option of "line in" but after climbing all over the interior I can't find the jack to plug the cable in and doesn't appear to be mentioned in the manual. Any ideas?
There will be a way to connect an external input. Probably only 3 wires required if it is only audio, a couple more if video too. You'll effectively be doing something like this:
except that the pins/plug on your unit will be different. You'll need a wiring diagram and to find that you'd need to tell us your year model as well - but now I notice this is in the 2016 section so there's a clue.
The Hitchhiker's guide to the the Triton universe and NTN.