Does the radio look like it has been out? If the previous owner had their own aftermarket unit, they may not have plugged in the aerial when they changed it back.
Does the display show a program is tuned, or does it run around not finding anything?
Have a look at the aerial on the roof. Try a new or different mast. The mast core is often carbon fibre, and can crack up inside. This is especially so if a mascot has been on it, causing it to bend in the wind.
Radio extracts with standard keys. At each end where the push buttons are, there are two blanks. Prise these off to insert the keys.
