1999 Peugeot 406 want to swop built-in radio/cass for cd player and retain facia style

if you go to a car audio retailer such as halfords they will be able to supply you with a radio blanking plate which allows you to fit a standard size radio if one is available for your car.you may also need radio harness adaptor plug depending on the radio fitted

try a reliable car breaker you may be able to get an origonal cd player for your pug for around £20/£30...hope this helps

The suggestion to obtain a secondhand radio/cd is a sensible one, however, later Peugeot 406's have a chassis coded radio which will need to have the chassis data re-written into it. Earlier radio/cd players will have a code or will need to be de-coded, the early models may have there own display or use the dash display and are not interchangeable.
The later chassis coded units also suffer with weak or faulty tuners and poor quality cd mechanisms, dodgy on/off buttons, cracked PCB's etc, you may be getting a worn out unit for your money which would be your choice.
The best solution depends on your budget and whether you have and or want to retain your steering controls for the radio, this will require a higher price radio and an interface.
An entry level cd/radio and a fascia adapter will cost you around £65+vat, for a quality brand name like Sony with steering control input, steering interface and fascia will cost about £110+vat. Fitting is plug and play on the early model 406, the later model will require checking before plugging in to make sure the wiring is correct.
Fitting by a professional for the later type is recommended, expect to pay between £20-£40 + Vat
Good luck, I'll happily answer any other questions that you might have.