Car that sits out

I have just purchased a 1966 Thunderbird with the 390. The body and paint is in decent shape. My garage already has a couple of classic cars, so the T-bird had to sit outside. Should I use a plastic sheet under the car cover on those rainy days as a water barrier? Some tell me that the plastic will act like a green house and keep moisture in. My though is to cover it with the plastic + car cover on those rainy days (thus keeping the water out of the window area, cowl and trunk areas). What should I do if this car must stay outside? Those 4-post lifts that allow one car to be stored above another are expensive. Thanks for your help,

Harold in Illinois