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I'd go back and ask them to show you what it was that they connected, and then unplug it and see if the warning light goes out. If it does then I'd guess you'll probably want to leave it at that and continue driving the car the way it is, since it's been running reliably (presumably) for the last year without that connection in place.

A battery warning light is something which you can't ignore, so if it still stays lit with that connector unplugged then you'll need to investigate further. You wouldn't be able to do that with any confidence as things stand at the moment though because the connector is an unknown factor, so I'd definitely go back and ask about it.