Hi Seb,
Welcome to the Heartland Owners Forum and Family. We have a great bunch of people here with lots of information and all willing to share their knowledge if needed.
I don't know if they are still putting the 12v outlets in for sure, but you need to look in bedroom where the tv is installed, you should find the antenna amp there. If they are still installing them they were usually in the same area. Or in the living room maybe on bottom of overhead cabinet where the tv is installed.
Maybe someone that has a 3450 will jump in soon and let us know for sure.
Enjoy the forum and your new Big Country.
Jim M