I am presently going through this routine myself after disconnecting the battery to recharge it, probably as a result of leaving the interior lights on. I made the mistake of pushing buttons mindlessly to see what results and got things out of whack. Finally read the manual to learn about the code card and that I would have to leave the radio on for an hour to get back to just the word 'code' being displayed since I had exceeded the number of entry attempts allowed. It did go back but I haven't got up the nerve to start over yet. I had also noted that the clock would not display and the manual said nothing about why that would be but I am hoping that when I enter the code and the radio is restored, the clock will come back as well. I am going to invest in a small 12v keep alive battery that I can clip on to the leads should I want to disconnect the battery anytime for charging, replacement or terminal cleaning. I had not known how dependent automobile systems had become on having continuous voltage. Also have a 2004 Thunderbird and the manual warns that if the the battery is disconnected, the transmission will have to relearn its optimum shift points. Oh well... One more reason to keep a small 12v keep alive handy.