In former times, young or remained young often changed - besides the exhaust system - the power of their vehicles illegally. Is not it substantially less harmful, with their interests shifting towards the sound in the car? Even if there are rumours of systems that kill passengers if they are operated with full power output - provided they are to remain in the vehicle's interior.
It is not easy to integrate an improved sound system into the car, not just because of the optical impression. The connections are available through ISO plugs, however they do not contain all signals. If the vehicle is equipped with a MOST-Bus or CAN-Bus it becomes even more difficult. For the CAN bus there is hope in the form of an adapter, which reads the position of the ignition start switch, the driving speed, the lighting and possibly even the operation via the steering wheel. The main purpose of course is to change the loudspeakers. What is your priority? Is it the total sound experience or the virginity of the door lining? For the latter the component systems with 13 cm diameters will usually fit. Tweeters with woofers and/or midrange drivers and a frequency switch in between will result in reasonable sound. Except for the bass range, the sound and the clay/tone quality are comparable to the larger systems. By soldering tin foil condensers into the plus wire, the sound may be improved even more. Vibrations should be tracked down, and eliminated by heavy damming material (e.g. from bitumen). There is an enormous selection of output stages. Depending on the location larger cable distances have to be covered. It is advisable to keep a distance from the original wiring in the vehicle, particularly from the (thicker) plus wire. The Chinchwire should be avoided, too, to be rewarded by the absence of turn signal clicking and other humming and/or whistling noises. Nevertheless, spurious signals can be eliminated by the installation of an anti-interference device. Naturally, the power supply should be sufficient; besides a possibly stronger battery (and maybe generator) the cable diameter should be appropriate. Improving the clay/tone quality does not only make the car more heavy, also the trunk is in vain. Here the same question about sound or optics emerges again, extended by the possible loss of trunk space. The size of the area that should be exposed to the sound plays a role, too. Limited to the front floor space probably only Subwoofers with 20 cm diameters are possible. Usually, you will build a Subwoofer crate from 16 mm strong MDF plates with damping mats yourself. If there is sufficient space in the back you may choose from the very rich offer the ones with approx. 30 cm in diameter. But you should take into account that these require a run time correction, to harmonize with the front loudspeakers. You should consider smaller Subwoofers, because it is easier to control them, as there diaphragm weight is only one third. However, the resonant frequency and the low bass ability decrease. A particular impression cannot be obtained with these loudspeakers because of the smaller maximum sound pressure. Due to the smaller efficiency it is rather necessary to have a stronger output stage. The durability of the diaphragm should not be an issue, because different materials can be used, among others, even carbon and kevlar fibers, which are as resistant as a bulletproof waistcoat.