I am the proud owner of a VW Beetle Convertible '79, a car you will never see in the US - a BMW Z1, plus Alfa Romeo Spiders, plus ... a P944.
So far all the cars either held their values or increased over time. The normal time span is about 10 to 15 years before a noticeable move in price comes. Patience! If you are just interested in a fast gain - don"t invest in a car! But it is definitely a lot of fun!