Check comment, and I simplified your code.
function makeSpecial()
if isVehicleOnGround == false then --this function must use arguments! Click on it (it will be highlighted)!
if getControlState ( source, "steer_forward" ) == "down" then
setWorldSpecialPropertyEnabled ( "hovercars", not isWorldSpecialPropertyEnabled ( "hovercars" ) )
elseif getControlState ( source, "enter_exit" ) == "down" then
setWorldSpecialPropertyEnabled ( "aircars", not isWorldSpecialPropertyEnabled ( "aircars" ) )
end
end
end