The short answer
Find and remove the source, then deal with the air conditioning, which is where stubborn musty smells usually live. Air fresheners hide the problem rather than fixing it.
- Empty everything and vacuum thoroughly, including under the seats and in the boot well.
- Check for the source: spilled drink in the carpet, something under a seat, a damp footwell from a leaking door seal or blocked scuttle drain.
- Damp carpets: lift the mats, dry the car out with the windows open on a dry day, and find where the water is coming in.
- Musty smell from the vents: the air conditioning evaporator is damp and growing bacteria. Replace the cabin filter and use an air conditioning cleaner.
- Bicarbonate of soda on the seats and carpet overnight, then vacuum.
What will ruin it
A persistently damp footwell is not a smell problem, it is a leak, and it will cause corrosion and electrical faults. Find it rather than masking it.
Why it works
Odours lodge in soft furnishings and in the wet surfaces of the ventilation system. An air freshener adds a smell to the air without removing anything from either.
Household chemicals can be dangerous in combination. Never mix them, keep the room ventilated, and test anything on a hidden patch first. Full disclaimer.