It's the plastic lense anti UV covering that yellows, so you need to remove that aged layer, wet n dry, G3, handful of gravel etc.
....the worst for going yellow were those MK1 Mondeos, they looked like my piss first thing Saturday morning after a heavy Friday night.
......talking of which, I can't remember the last time I saw a MK1 or MK1 facelift Mondeo, all bangered and made into baked bean cans now.