Thanks for the discussion, everybody. I am about to dismantle my dash to address some other wiring issues, so I will make the change to the reversing lights that JayDub has suggested. I am also replacing the courtesy (festoon) bulbs and indicators with their LED equivalents (new flasher unit also required for LED bulbs) so will soon have a (partly) 21st C Fulvia. While I respect Lancia engineering highly, I am much less purist when it come to Italian electrics.
While we are on electrical & dashboard issues, Tim (LanciaLulu) has previously suggested using ceramic bullet fuses, which I did not know existed. I guess they should have a higher melting point and lower thermal conductivity than plastic fuses and so might reduce the amount of heat that reaches, and can melt, the fuse cover. 16A ceramic fuses can be bought from Frosts for £2.65/20 (
https://www.frost.co.uk/torpedo-fuses/).
I have also discovered a good supplier, Boltbase, of small quantities of bolts:
https://boltbase.com/. Very useful for small quantities of specialist items - like grub screws for knobs and small domed, countersunk bolts for lock cover plates and sill covers).
Glyn