A line converter is a solution when the original car player has problems installing a car stereo. In some cars, changing the player has been made virtually impossible (a close look at Volvo).
There are two types of line converters: passive and active. In practice, passive is the cheapest (and worst) solution for a few reasons. First, the sound quality is not at all at the level of the active line converter. Passivity also makes it more sensitive in car hi-fi to bad interference coming through the speakers. The lack of forging control is also bad. Thus, when using a passive line converter, it is not possible to adjust the volume of the signal from the car sound system. One more problem is that there is no excitation current to the amplifier then. Then there will be another additional problem to solve, which is not a good thing. Then there is one last problem, which is that the passive line converter may not fit the car at all.
The active line converter, on the other hand, is a little more valuable, but it also adds value and ease to the installation of a car hi-fi. Some models have an automatic excitation current for the amplifier. The connection is a bit slower when the converter needs a continuous current as well as a ground connection in addition to the speaker signal. The sound quality, which is certainly in the mind of every car hi-fi installer, is many orders of magnitude better than the passive one, which is already a really important point.