If you're getting 5v at the motors then you're dropping voltage, logically. Either the switch contacts are faulty or the motors are faulty, causing too much resistance.
In your situation I would be looking at the switch since no windows work.
Grab a halogen bulb test light and verify it lights up at the motor connector, circuit voltage means nothing without the ability to carry current and current verifies a good ground.
You live in Illinois where rust/corrosion is common.