Plastic headlamp lenses (not "covers") are required by US law to be coated
with an anti-abrasive epoxy. It is this epoxy which gets yellow and cloudy
with time and exposure to sunlight.

The only cure is to remove the epoxy entirely, then replace it with a new
coating.

I've never actually tried the commercial preparations myself, but I
understand that, properly administered, they are effective in restoring the
headlamps. Auto detailing places often offer headlamp restoration services
which involve epoxy removal and renewal.

I suppose you could simply remove the epoxy entirely and just polish the
plastic, but the underlying lens may not be optimized for use without that
coating. If you want to try this approach anyway, Brasso metal polish may
dissolve the epoxy sufficient to restore clarity to the lens. Brasso is
excellent for removing scratches on watch crystals and those clear
instrument panel covers on your dashboard. A caveat: I'm told that Brasso
and other such potions can embrittle headlamp plastic, which may make them
more prone to stone fracture. Brasso is available at any grocery store.