I had exactly the same problem with an old Ford that I loved as soon as I saw it. What you need to do is, while we have some decent summer sunshine (if it comes back), try to leave the car standing for as long as possible in bright direct blazing sunshine all day, with all the doors and windows closed, to raise the temperature and really "bake" the inside of the car intensely. Do this for several days in a row if you can. Then the next few days, do the same but with the sun-roof or window open to let the evaporating moisture in the air out. The exiting dampness will take the musty smell with it, and after that just keep it well-aired with as much fresh air as possible, especially on very hot days with the windows open. Ohh.. And just make sure you remember to let it cool off a little at the end of the day, before you sit in it and touch the wheel..!!