just got a tune up on my previa it starts and run good except when it rains it

Get a spray bottle with water, spray a MIST around the plug wires and cap while the car is running. If the rpms change or runs rough you have a defective part.

Return to place of tune-up for 'ignition sealer' treatment, shouldn't be any charge if it's anything to them! .. OR, Google > ignition sealer... Do it yourself... Possibly missing//damaged rubber gasket under distributor cap.