All LED's are DC and driven by a driver depending on the type of bulb. Some are constant voltage and some are constant current. The drivers come in both internal and external versions. Most consumer LED's are internal driver types. The drivers regulate the voltage and or current from the supply to drive and protect the bulb. When AC power is used the driver also has a rectifier circuit to provide the DC. With your test on the battery and the lights working and a voltage only test on the tractor you have a current issue. It could be a corroded connector, a mostly broken wire or a switch with bad contacts.
Jimmy
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