How can you fix the car radio of a 1999 Mercury Mountainer when the clock doesnt show nore the radio station?

This won�t be as easy as you think, it all depends on what kind of digital display that radio has. Can you still hear the radio stations? Can you still change the stations although nothing shows? In most of the older radios the display is illuminated with a back light located somewhere just behind the display itself. Taking the radio or face apart will expose behind the display thus locating the 12v bulb or 2 in order to replace them. Before assuming the bulbs are bad you should remove them and test them across the leads of a 9volt battery. Generally the bulbs could be replaced with any other bulb as long it is similar in size, you could also customize it by changing the color just make sure that you are using a 12 volt bulb. If you do not find the bulbs then your display uses another system to illuminate its numbers. Perhaps the numbers themselves light up? In any case this is much more difficult of a problem to solve and may be needed to take it to a tech for further checking. You could also check for bad solder points (cold solder) Cold solder points are solder joints that are lifted from the PC board, they may not be so easy to spot but in most cases will have a hair line crack or ring around that connection. Resolder those places if spotted, loose wiring or connections. Hope some of this helps.