grand cherokee jeep radio problem?

my grand cherokee jeep year 2000 factory radio has recently become blocked by a roadside repair service mistakenly entering the wrong code three times in my service manual it states that to correct the problem i must switch on the ignition and radio for 30 minutes during which the display will read wait ,which it does ,however the message does not change and i cannot re enter the right code which i have can anyone tell me what the problem is i have tried disconnting the cars battery but this does not help , i have spoken to a crysler mechanic and he has told me to drive the car for two hours with the radio switched on and the wait message should change ,but this seems a bit accessive