Big big delay ....... but finally, here it is ..
When i bought my used car, the SL antenna was already installed. FM reception was very very bad. No secret : a small antenna gives bad reception.
I've decided to extend the antenna in the rear bumper using a simple cable. Even if it's not "perfect" (based on radio reception theory), the result is good enough for me (FM reception).
The idea is to add a new antenna, which is connected on the antenna amplifier. This connection is done inside the amplifier itself : the new antenna is soldered on the main PCB. Therefore, some electronic skills are needed (at least to be able to solder a wire on a pcb).
Of course, the amplifier warranty is removed ..
A) The antenna amplifier is located in the rear left side of the trunk (See "Open side trunk paneling" thread)
B) Disconnect all the cables (3 for Europe as we haven't Teleaid system). The connector from the antenna is difficult to be removed.
C) Remove the bolt of the antenna amplifier and remove it from the car.
D) Unscrew the two screws on the amplifier, and open the box
E) i've used the teleaid connection (which is empty in Europe) as a hole for my new cable (C) (Small Antenna coaxial cable). The shield of the cable is soldered on the PCB ground (B), the hot wire on the FM antenna connection (A).
Final touch, some epoxy glue (D) is added in order to secure the cable
F This new cable goes out of the car through the left ventilation opening (below the amplifier itself (the new cable is shown in green in the following pictures)
G) To go further, the rear bumper shall be removed (i've made a thread on how to remove the bumper).
Outside the car, there is a connector between this new cable and the "added antenna". The added antenna is a wire, 3 meters lenght which is glued on the internal side of the bumper. This is a simple copper wire (not shielded and no need to be big). The only important thing is that the end of this wire shall not be grounded (shall not touch the metal part of the car itself). The cable goes from left side to the right side of the car on the top of the bumper and from right to left.
I hope this explanation is clear enough.. If not, i can add some precision or bigger pictures.
The result is good enough for me : good FM reception as well as TMC channel.
It was easy thanks to the fact that Teleaid system is not installed (no hole was needed in the amplifier housing).
As an alternative (if the teleaid system is installed and you do not want to make a hole or solder somethning in the amplifier), the cable from the amplifier to the antenna can be cutted and connected to this new antenna.