HD Set Top Boxes - Cables

Just got an HD set top box for Xmas, all plugged in and ready to go...except that the red, yellow and white cords to connect into the TV, are into the same inputs as the WII (which has the same style/colour cables). Does this mean that each time I want to use one, I will have to disconnect the other - or is there some sort of adaptor to allow me to plug both in at once?
If there is no adaptor, will it also mean that I lose all of the scanned HD channels each time? Sorry if these seem ignorant questions, I'm not the most tech savvy person around at times.
If there is no adaptor, will it also mean that I lose all of the scanned HD channels each time? Sorry if these seem ignorant questions, I'm not the most tech savvy person around at times.
