Back in the dark ages I had a Rover 214 with a tow bar. But it didnt have
an indicator to show when you had a trailer attached.

It was possible to buy a replacement indicator relay which had an extra
output to light a bulb on the dash when it detected that there was an extra
bulb in parallel with the indicators.

How would this have worked? Would all of the indicator current have been
sent through the coil that activated the switch for the extra contact, and
only when the extra bulb was connected would the current be sufficient to
throw the switch?

Or something else?


--
Chris