To find out if yours has one, you'll need to:
1. Check your owner's manual: This is the best place to start. It will specify if your model has a cabin air filter and give instructions on how to access it.
2. Look behind the glove box: If you don't have an owner's manual, carefully remove the glove box (usually involves removing a few screws or clips). Look closely behind where the glove box was; you might see a filter housing.
If you don't find a filter housing, your Caravan likely didn't come with one from the factory. Adding one would require aftermarket installation, and that would involve some research to find a compatible filter and potentially some modification.