People often refer to a headunit as a CD receiver although others just name it as a radio. If there is something wrong with your current headunit, or if you just want to improve the sound output inside your car, then you might consider replacing it with a new one. 

But hey, before you rush to the market and shell out money for that new headunit, try to check your old one first. Find out the size of the headunit is it two inches tall and then seven inches wide? This specification is that of a single DIN head unit. This is the most common size for head units. If yours has a smaller size, buy an adapter so than the single DIN head unit can fit in. If it has a bigger size than that of a single DIN, then you must find one that specifically fits the compartment.

 Also consider the functions that you audio system can provide. Does it have a radio? Is it iPod ready? Does it come with a CD, a tape deck, or both? Ask yourself what kind of media are you inclined to use more. If  you are planning to add new kinds of functionalities to the headunit , you might as well find one that is upgradeable.

Remember to always test the product that you are considering. Try getting your fingers on the buttons and check how it feels is it firm? Is it susceptible to breakage? Test most of the keys and also the radio. Does it easily pick up signals? If it has slow pick up rate, better say goodbye early on that than end up regretting the purchase in the future.