It's probably more of a amperage than voltage issue now. Those reducers, at least the ones I've seen, are designed for light duty loads like gauges. You'd probably be better off with a ballast resistor to be honest but I still think it would reduce the current below the level needed to make the motors work. Will they work if you give them a hand starting?

They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.