There are a few car radios where the below info is not true but they are rare....generally, car radios have very low cost, and poorly designed AM radio sections and no connection to the external antenna
For most of the current major brand car stereos, the External antenna has zero effect for Amplitude Modulation (AM). The car radio's AM antenna is inside the radio, it is a ferrite rod with thin magnet wire coiled over it. The reason so many car radios do not receive well on AM is the antenna is inside a metal can (the radio) and again inside another metal can (the car)... Generally, the only thing your external antenna provides is the Frequency Modulated (FM) signal. In this case...if you get good reception with the stubby antenna, then a longer antenna will not do much...the opposite phenomenon is true...poor FM signal, Stereo Mode (ST) dropping in and out?...change to a longer antenna will help some
I do not have a smart fortwo yet...once I get mine I will take the Visteon Radio 10 out and explore all of its design... if as reported here it totally sucks... I will probably add in one of the better aftermarket head units with more capabilities
In the past I have had great success with removing the AM ferrite antenna from a car stereo and installing a much bigger and better antenna from a cheap garage sale Short Wave (SW) radio... I have also made my own AM antennas but found that the garage sale 1950s~1970s SW radios are about $5.00... I can't make a AM antenna that cheaply...
If you are real big into AM talk or rural farm reports kind of listening... I can tell you this...most of todays modern aftermarket car radios really suck... gone are all the old Delco rock solid radio receivers
KD5NCO