How Do I fix the Radio Reception on my car?

I have a 2003 Mercedes-Benz S600. I bought it very recently and the car had no
amplifier so the dealer said he'll put on an amplifier in the trunk of the car (assuming
thats where it's suppose to go). The car had its original 2003 Mercedes command
system removed and a 2011 pioneer command system installed. Everytime I try and
navigate through my radio stations through the new command system, I noticed that
the car has AWFUL radio reception and it is literally all static or a really bad signal. I
also have a 2005 Mercedes-Benz S500 with its original command system, and a 2004
BMW 745li with its original iDrive command system, and both cars radio reception is
fine. Both pick up good signal of local radio stations where it is suppose to pick up a
signal but my S600 can't. What do you think the problem is? Is it the amplifier that the
dealer installed a bad one? Is it the Pioneer command system the can't pick up the
signal? Or is it something else in the car?

PS: The S600 has no antenas that I can adust, so if you can tell me where the antenna
of the car is I'd greatly appreciate it. Also, if I'm trying to tune a station, it seeks it in
HQ so I don't think thats the problem. Thanks alot for answering!