It depends on the the area you live in and how strong the radio signals are and also if you're installing a DAB (digital audio broadcasting) radio. To be sure to cover ALL options the radio should have an auxiliary input, to allow you to connect your smart phone to the radio, thus giving you internet radio on your car radio. The best position for the aerial on most vehicles is above the interior mirror, pointing back at 45 degrees. This gives minimal loss of signal from the aerial to the radio, relatively easy routing for the aerial lead, is above the height of the car's meal structure and if slanted at 45 degrees backwards will be minimally affected by the engine interference, if any. Also (odd one, this!) it can be quickly folded flat on the vehicle's roof for car washes! DAB aerials can also be fitted there, but they have to be fitted vertically. They are normally much shorter than ordinary car aerials anyway.
Hope this helps!