My radio has all the bells and whistles. It is a large square box that an aftermarket radio will not fill. It seems to have no reception at all. when it gets warm it starts to recive but very weakly. To change the antenna would be troublesome as it is in the rear and I dont feel that is the problem. It also has theft alarms so will a used one is I can find it work. I would like to lose it but how to fill the hole

A used one will work, you can get the code at the dealer. As for aftermarket radios: standard aftermarket radio size is referred to as 1 din. Your stock stereo is 1.5 din. To install an aftermarket stereo in Amy car requires an install kit, in your case the kit will make up for the size difference by just filling the gap with black plastic. But there are aftermarket stereos that are 1.5 din. They will still require an install kit, but will completely fill the hole in your dash. If you're planning on getting a $99 stereo at a parts store don't bother and just find a stock one. But spend $150 on an aftermarket stereo and you'll be very happy. Good luck.