Power tailgate override

After checking the wires as Mr. Sinister said, if they look okay, use a battery or a small battery charger to lower the window. Negative to a good body ground on the tailgate, and use the positive lead to probe the two wires at the connector. One is "up", the other is "down". It wouldn't hurt to clean and lube up the window track a bit while you're inside there checking the wires, too- it's probably pretty crusty after 50 plus years.