Guys often confuse "ram air" with "cold air". They are two different things actually.
"Ram air" is pressurized air, boosting horsepower by feeding the motor denser pressurized air. To catch pessurized air that will increase horsepower you need two things, a scoop that sticks up high enough to catch "clean air" and enough forward velocity (speed) that the air pressure will be significant. Unless you are traveling real fast, don't worry about "ram air". If you stick a scoop 12 inches above the car, the horsepower gain at 60 mph would be negligible, certainly not worth putting up with an ugly scoop. You've all seen the Pantera roof-line scoop that travels up from the motor, bends around the roof of the car, follows the roof line forward and has an opening right at the top of the windshield. The air accelerates as it passes over the windshield, that scoop takes advantage of that. OK for a race car, but I doubt many of us would want that contraption on our pretty little Italian sports cars. The other drawback of "ram air" is that its biggest advantage only occurs at wide open throttle. If the throttle butterflies are partially closed, and the intake manifold is under vacuum, the air being supplied to the motor is not much denser.
"Cold air" takes advantage of the fact that cold air is denser than warm air, horsepower increases 1% for every 10 degree F drop in intake air temperature. Cold air is not dependent on the speed of the car or the position of the throttle butterflies, therefore cold air will help any vehicle, including vehicles driven by old slow guys like me. Cold air can be picked up anywhere, but I would advise against ducting cold air from the rear of the car, because the rear of the car has a tremedous low pressure zone behind it when the car is moving forward. A simple NACA duct on the side of the car would work perfectly; NACA ducts are available from race parts suppliers like Pegasus.