You could try Agenta Audio (www.agentaaudio.co.uk). They cover in-car radio and audio electronics from the mid-1930’s through to the late-1970’s. Services include repair, polarity conversion, concours-standard restoration work and upgrades to more modern systems.
Reader Jeffrey Bridges, a technical adviser for the 6/90 section of The Wolseley Register recommend Adrian James (regiesradios@gmail.com). He said, 'In 2017, he converted the original Radiomobile valve radio in my 1958 Wolseley Six-Ninety to work with FM, to negative earth and also to take a USB input. The service was excellent, if slightly more than the £45 my father paid for the radio in 1960!'