No. Diet drinks do NOT use corn syrup as a replacement for sugar. They all have artificial sweeteners, and no research has been done that I can find on the effects of those chemical on guinea pigs. I wouldn't give my guinea pigs a diet drink for anything.
There's also considerable controversy over whether cranberry juice does anything at all for guinea pigs. In humans, the theory is that because it acidifies the urine, it makes the bladder a less hospitable place for bacteria to grow, and thus helps prevent bladder infections. But guinea pigs are herbivores, and as such, they have alkaline urine. You'd have to give a HUGE amount of cranberry juice to even stand a chance of acidifying the urine, if it's even possible. Their bodies would be working to keep the urine alkaline, and it might not even be possible to get the urine to an acid state.
I personally think cranberries are useless for guinea pigs as far as urinary tract issues are concerned. They're a pretty good source of vitamin C, and a lot of guinea pigs like them as treats (don't use the heavily sugared ones). But my own opinion is that you're wasting your money if what you're trying to do is prevent bladder infections. Keeping the cage clean and the pig's bottom dry is a lot more effective.
