First off, you need to remember that different people store fat in differnet places. So for you it may very well be quite natural for your body to lose it's fat around the stomach and back last, and conversely gain it there first as well.
If you've hit a plateau with cardio then there a few things to look at. Is your cardio intensive enough for your goals? Try mixing it up and throw in some high intensity intervals. Remember: if you do what you 've always done, you'll get what you've always got.
Take a look at your diet, as it's just as, or likely more, important than your training. All the running in the world won't help you if you're diet is crap. Hit up a macro nutrient calculator and keep a daily meal log until you understand your food. Eat clean to get lean... that sounds ghey but it's very true. I won't get deep into nutrition here as it's a vast subject but take the time to research how to eat to drop unwanted weight.
As for developing muscle in your core, realize that all the muscle in the world won't show if it's buried under fat. To build muscle you want to look at much better exercises than crunches or situps. Full body movements like mountain climbers, dumbell swings, jump knee-ups, planks, etc. are much better for developing core. Ideally you would want to integrate SOME core into your workouts about 3 times a week. Don't over do it, but steady, consitent work load is the way to go.
Anyway, sorry to be long winded. First thing you need to do is figure out your diet and get STRICT. There's no way around that. And by strict I mean knowing how many calories you can eat a day and still lose weight, and what types to eat and when. Just goolge it, it's overwhelming at first but you need to learn it. Bodybuilding.com is a good place to go for this type of thing.
"I do not pretend to know where many ignorant men are sure" - Clarence Darrow