I've managed to get somewhere on 1.8 MHz from home, by plugging in just the centre of the coax that goes to my HF wire. The Z-11Pro tuner will tune that and I was received on WSPR by G6WRW in IO82UJ (Kidderminster, 200km). I was -26dB S/N, with 10 watts, so I only just made it.
There's S9 of noise here on 1.8 MHz and I'm receiving nothing. I thought the computer might have shut down or something with that much RF only a few centimetres from it but it wasn't affected.
Found a site with circuit diagrams for the Ham International/Hygain/Tristar SSB CB rigs.
Stalker IX CB with circuit diagram (this is the AM/SSB only model, not the one with FM but it's mostly the same). Lots of other information on CB rigs on that site.