Remember that it's stereo, not surround. When listening to a home stereo, you probably noticed that it sounds best when the speakers are of equal distance from you, that you start to lose the stereo effect when you get closer to one speaker and further from the other. This is already a problem in the car with speakers in the front door panels (close to one but far off axis, far from the other yet much more on axis), but did you ever notice how close the left rear speaker is to the driver's ear? The way I sit, that speaker is even closer to my ear than the door speaker. This is important because humans are very good about determining the origin of a sound source, based on arrival times. What this means is that the sound from the rear speaker may reach your ear before the front speaker, which leads the mind to think the source of sound is behind you. While some people suggest just using the fader and balance to adjust it, that doesn't work because you can't trade time for amplitude. So, if you must have rear speakers and won't be playing them loud compared to the front, then it doesn't make much sense to spend the time, effort & cost to upgrade them.

Let's start with what "rear-fill" is really intended to do. When you're at a live performance, even though the performance is in front of you, some reflections of the sound come from behind you and lend an ambiance effect, such that you get a sense of the environment, the space the performance happens in. There's no direct sound behind you, only reflections from the direct output that originated from in front of you.

In a car with only front speakers, the direct sound from them reaches the rear of the vehicle, and is then reflected back to the front, like it does in a concert hall, enough that there's often no need to implement rear-fill speakers.

If a person wants their system to mimic a larger space, such as a large concert hall in a big vehicle, then a rear-fill solution might be warranted. The thing is, you can't just put speakers behind you and get the correct effect, because that doesn't do a decent job of reproducing the desired environment. In the concert hall, the sounds reflected off the rear wall are delayed relative to the direct/original output, they're bandwidth limited, and attenuated.

So, the correct way to implement rear-fill is with midrange/midbass speakers only, limiting their output to 3kHz at 6-12dB/Oct., such that their frequency response rolls off at the high-end, just like at a live venue. They should also be at least 6dB (9dB is often better) less SPL than the front speakers, to mimic the attenuation caused by distance. Since the car is much smaller than a concert hall, the output of the rear-fill speakers needs to be delayed relative to the front, at the rate of approx. 1ms per foot of distance desired. Also, rear-fill doesn't have to be stereo, mono will work just as well, if not better.

I've yet to see anyone here implement rear-fill this way (mids only, 2nd order 3kHz low-pass filter, ~10ms delay, -9dB SPL relative to front), but there have been many winning SQ competition vehicles over the years that had such implementations. It's a lot of time, money & effort to get right, so it's no wonder that most don't bother.