Typical fault with cabriolets is after years of opening and closing the boot the cables give out, so the radio is probably fine and replacing with an after market will give you the same or even a worse situation. It might not be the best solution for you but first of all test the cables from the boot to the radio, remove the boot trim and find the leads from the antenna amps, should be two of them that go to the radio, I would guess a mustard looking connector and possibly a white one. Use a multimeter to check for open circuit back to the radio end, no easy I know as the cable ends are very far away from each other to bridge with a meter. If you decide they are faulty you can repair them by cutting out the section through the boot hinges and feed two new pieces through and join them back up.
You may be lucky and the test will say they are OK and then maybe you need a new antenna booster for the boot. Or the power to the booster is dead, pretty sure it has a +12v feed rather than getting it through the antenna cable.
Not an easy job, but I cannot see another way for you to fix the reception in your car.
