There should already be a connector in the dash for one of the radio systems (Harmony, Symphony, etc).
Are you sure it's a Symphony? Got a pic of it?
The Symphony doesn't do TV - and it needs the switch pack YUB100591PUY to do Low Line satnav (shown here).
The switch pack is #5 here -
https://rimmerbros.com/Item--i-GRID001635
Nos. 2 and 5 show the Symphony without and with the switch pack.