Mouldy car

[Clobbered]

Help! DS has inherited an elderly car that is parked on our drive for the duration. We are taking it out for a run once a week to keep it going. I've noticed a bit of a damp smell for the last few weeks and today there was a LOT of condensation on the inside after I had driven it for about half an hour with the heater on. I'm thinking there must be moisture trapped in the seats as they feel a bit damp. There is a patch of mould that has appeared on the back seat and seat belts..
How do I get rid of the mould?
How do I get rid of the lingering moisture?
If I can clear it up, I plan to sling a tarpaulin over it to stop any more water getting in.
Any other suggestions?

don't put a tarp on it, you'll just get rust as well as mould!

TBH it does cars no good to sit on drives. Any reason you need to keep it?

two common reasons:

heater has been set to "recirc" so it does not suck in fresh air from outside to force the damp air out (this will be the cause for people who can't see out on rainy days)

or

water leak, most often round a window, letting the rain in. Feel the carpets all round, and in the boot. Sometimes it is just the drain holes in the bottom of the doors that are blocked with dirt and can be poked or hosed clear. Otherwise you will have to take off the door panel nd look. Sometimes it is the plastic sheet that has been removed, damaged or misplaced, it is supposed to lead and water down inside the door so it can escape.

Most UK cars that are still on the road are made of Zintec steel which is rust-resistant, provided paint damage is repaired. A badly rusted car is usually not worth repairing.

BTW running a spare car for short periods does the engine no good. It needs a long enough run to get it thoroughly hot. Especially bad for diesels.

Thanks for all the advice, will digest and apply!