radio code

Hello every one. I have just bought a jeep grand cherokee and I have a problem with my radio. I turn it on and it is OK for only 5 minutes after that it requires the 4 digit code. Unfortunately I don't know its code and when I asked at a service store for unkocking it, they would do it towards a very expensive price. Does any one knows if there is an universal code for all original radios of Jeep Grand Cherokee '97? Or is there any calculator program that does it? Please do help me. I am very found of my new car and I can't enjoy it properly without my radio

Thanks mates