The reason why people put money into their cars is usually not for capital gains but rather the status which accompanies it. It's not to say you can't make money out of modifying cars- but I think you have to have a bit of credibility (and funding) before you'll be able to start making a lot of money out of it (eg. companies like GReddy, AC Schnitzer and Brabus). These companies typically import the vehicle themselves, apply their own modifications, and then resell. My cousin purchased a 2003 modified GReddy z33 Fairlady (aka 350z) for around $100,000aud (RM308,000). The vehicle was second hand, then GReddy (or the local distributor) imported it into Malaysia, applied all the modifications (eg. GReddy Oil Catch Tank, Strut Bar, Spring F/R, Power Extreme Ti (Titanium), Timer, various meters and control units, Airinx Adapter, etc..) and tuning.
... but most people aren't that pro in terms of resources and so on, so I'm not really sure how much money you can make from it. If you're lucky you'll end up with a little profit for your effort, but then that profit will be eaten away by the fact that the car has mileage and is second hand, has no warranty/guarantee, etc... lowering the value of the car. The aforementioned z33 at least comes with warranty for all parts and servicing.
The other way to make money is buying cars before they go out of supply and then when they are in high demand, you can charge a premium for it provided it's in near-new condition. Quite a few people pulled this stunt off with Golf GTIs and the new Toyota Prius- selling second hand (but barely used) cars above the RRP.
... also forgot to mention the b/s of classic cars and so on, but then these are hard to find, hard to sell, and I don't think you're really interested in that.