If the amplifier in the boot really drives all of the speakers, it might be one of the ones which takes a low-power input. Something between line level and speaker level. A lot of cars do this.

If this is the case, you might be able to find a small booster interface which brings the empeg's line level outputs up to the "almost-speaker-level" outputs the factory amp requires. For instance, here is an example of the installation of just such a device. This particular one doesn't have a Peugot plug, unfortunately, but perhaps there's another manufacturer who makes a Peugot-compatible one?