Thread: Trailer Relay
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Harry Bloomfield, Esq.[_2_] Harry Bloomfield, Esq.[_2_] is offline
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Default Trailer Relay

Chris Holmes explained :
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?

Yes, through a relay with a low resistance coil winding.

They used the same idea for detecting when a car lamp failed long ago.
It's all done by electronics now.