Like Wayne said, the White Lithium grease solves this problem too. I had the same issue a few years ago and sprayed it twice. The first squirt lasted about 6 months and the second spray has lasted me about 3 years and still no squeak. I'd be afraid duct tape would fray at some point and bind up the window keeping it shut or gumming up the lock and not locking shut at all....

Since both approaches work, it's your choice.