How does electrical interference travel?

[GK5268]

I can't advise on the electrickery of the interference, but I had similar issue a few years ago when I converted all bulbs to LED, had a similar issue where the warning light indicator continuing to flash after the turn signal was cancelled (and become more rapid...) and some other weird things happening, such as front side lights operating when the brake lights came on. At that time, all the wiring was new, all well earthed, so not a wiring fault.

I fitted diodes to the problem areas, resolved them perfectly. Diodes only allow the current to flow in one direction.

I also have suppressed HT leads to prevent radio interference, the car could definitely be heard on analogue receivers, not sure about digital.