Firstly, musty smells usually originate from excess moisture and humidity - which means it's possible that mildew is growing in your car if there's no air circulating. In that respect, we'd recommend checking for any water leaks and give the car seats a good clean. Autoglym offers a good upholstery cleaner (amzn.to/2W5MhvM), as do Simoniz - which is also half the price (amzn.to/3c4L2md).

As for getting rid of the smell, Autoglym do an odour eliminator (amzn.to/3clxK4I). As do Turtle Wax (amzn.to/2YCPTHh) and Meguiar's (amzn.to/2WwDdiv). All are decent.