Voltage Converters are easy to make and a huge profit to the car audio business ... All you need is a non-inductive resistor for a load to suck up (waste) the current output and a simple resistor voltage divider to drop the voltage to line levels ...
I realize this isn't a solution for most people but anyone with even a basic knowledge of electronics and some basic soldering skills can easily accomplish it for the cost of a bit of shielded cable, an RCA jack and maybe a couple of resistors per channel ...
I usually hack my own line level outputs by taking the signal from the output of the volume control because a line converter can induce distortion and can hinder frequency and even phase response, especially if the manufacturer takes the cheap way out (and many, if not most do) and use a wirewound (inherently inductive) resistor instead of a non-inductive one ... the cost of including line level ouputs on any car stereo is no more (and usually less) than the cost of including a headphone jack in a home or the cheapest portable CD player