Plastic just fades over time. Trim restorers are available. Products such as Black Wow or Solution Finish will restore the appearance for a few months. Regular reapplication will keep the plastic hydrated and looking good.
For the interior, my personal choice is Meguiar's Quik Interior Detailer followed by a protectant. Meguiar's QID has a light cleaner built in so I will use that. It will leave the appearance looking OEM and deposit a small layer of UV protection. I will follow with a protectant to rehydrate the plastic. I have been using 303 Protectant. Meguiar's Natural Shine or Mothers Protectant works well. Regular application is key. I will use this on the plastic, rubber and vinyl areas.
For leather I will use a leather cleaner and conditioner. The factory leather is coated so one is essentially treating the coating on the leather and rehydrating that. Meguiar's has their Gold Class line available over the counter. Mothers has their leather wash and leather cream available over the counter. Applying regularly is key here as well.