Re: really desperate!
You'll probably find that your Guinea Pigs like to pee in particular parts of the cage, usually in a corner or near one side, while the poops are usually more spread about. If you put your litter box where they normally pee and fill it with some of the bedding they have already pee'd on hopefully they'll use the litter box.
My piggies liked to wee on the carpet of their ramps, however we set up a litter box with pine bedding and hay in it in one corner and they have started using this - (messy things sit on the hay and poop and pee on the hay as they eat it).