For appearance and modern day use in a vintage vehicle.
That is adding an FM IC inside somewhere rather than RF converter plugged in
aerial socket.
Assuming there is somewhere around the volume control to switch out the AM
and switch in an FM IC o/p audio . How to secrete that switch and a step
through the band switch as channel changer.
Maybe possible to change the LW/MW switch to MW/FM and if some part of the
dial scale is flexible then hide a switch behind that.
Any other ideas without affecting the external appearance of the original?

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Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/