Seat Ibiza mk3 - smelly air con - elekie&a/c doctor
Cabin filter is a good place to start . Should be behind the glovebox . Access may be from below , passenger footwell . Clean out and leaf debris while you’re there .
If that doesn't sort it then when you are next out and about run it on hot for a good twenty mins, the longer the better. It will drive out any damp which might be encouraging mould. In my previous car it smelled like a dead rat if I didn't use the air con for a month or so, even with a good filter but running it hot for a while always fixed it.