I’ve just discovered that you can use the Night Heater controls to switch the radio back on. Why they put it there is beyond me.
With the keys removed, hold down both of the buttons on the right hand side of the Night Heater control (the removeable type), the buttons being one above the other, and after a couple of seconds, the radio comes on.
I’m not sure to which marques of Scanias this applies, but it certainly works with the newer ones.