Why would a 1996 Plymouth Voyager make a weird squeak under the drivers seat when you accelerate or brake but only only when the weather is sunny not if it has been raining?

Sounds like the lower A-arm or the sway bar bushings , these are made of a rubber type compound and will squeak when they dry out . The squeak disappears when its raining because the water is acting like a lubricant . replace the bushings just don't lube them, lubing them is only a temp fix and they will start squeaking again soon , replacing these bushings will help with the cars handling.