Decorating a car for a wedding is actually a pretty easy task, and usually it is done by experts. Being an easy task, it might as well be done by you. You could decorate your car using fresh flowers you bought (not recommended, since fresh flowers exert fluids), you could use balloons (if you are careful to tie them well and keep them from blowing up) or use noisy cans tied in the back of the car.
Wedding car decorations are often done by flower decorators or wedding designers that tend to include this task among their other services. Their natural tendency is to use fresh flowers or colored ribbons.
However- the best decorators are the ones who specialize in this field and that decorating is their core business. Those specialists know their art and they tend to create a coherent aesthetic line between the cars shape, brand and color and between the decorations they make.
Do not forget to tie the decorations well to the anchoring points in the car, otherwise the decorations will be "gone with the wind", and it is essential to ride in a moderated speed so that the decoration might last the whole evening.
If you would prefer a more professional and impressive creation, you could use a car decoration service that operates in the customers house. It might be more expensive, but the added value and the impressive outcome are worth every dime. It's not so hard as it looks. You should try. It's great fun