According to me and many people, the installation of speakers in the rear sides is completely pointless ... The radio you have has a maximum of several W in RMS per channel, not 4x45 so after connecting a loudspeaker, which has 60W in RMS, for example, it will be worse than on serial speakers, which were originally in the car.
If you really do not want to go in a separate set + amplifier, then give yourself any modifications and buy the tired tires that were originally in this car.
A friend has a Golf IV and the front is made + amplifier without a subwoofer (DLS R6A) and to be honest, the legs fly like in the wind and the bass is really big ... Of course the door is properly soundproofed. I would go in this direction.