Thanks for the info. Next time I get a chance to get the radio out I'll have a look for FML and FMR. I guess the best place to solder to either side of the cut will be an existing solder point on the board, rather than trying to solder on the PCB track?
I'm wondering whether a DPST switch would be sufficient? If I'm only using it to make/break the FM signal line to give me 'radio silence' then I'm guessing single throw is fine... Then a separate cable will be connected to the test/aux in line I've (hopefully) found. For info, this test input has three pins, which apparently fit a 'servo cable' like these: link. I could then attach the other end to a 3.5mm plug/socket.
Having said that, if I find the FML & FMR, then in a way I might as well just do your solution as written in the first post, and switch my aux into the FML & FMR lines using the switchable 3.5mm jack like you used, that way it's quite neat in that when the aux is plugged in, it's active, and when it's unplugged it's FM. However having a toggle switch with the separate aux in gives me more flexibility to run a longer aux cable, and potentially have my phone/ipod plugged in even when listening to the radio, then just switched as desired.
Decisions...!
Thanks for the offer of help, too. I'll see how I get on when I next have the head unit out.