^major pain, and then there are all sorts of other issues (are you running independent amp? If so how do you get signal for said speaker. If you’re using stock amp, then why bother. If you’re aftermarket head unit, it’s not going to send out a 5ch signal. But most importantly, why is it even there? The answer, to make up for lack of quality elsewhere and to make things blend better, you put better stuff in you don’t need it as much).
I really think you should consider the front only approach. Do you have kids in the back seat or something playing video games / movies? If not then I really don’t think rear speakers are worthwhile and in fact damage the quality of sound up front.
It sounds weird at first, but listen to your stereo with front only on for a week or so, and then go back. See if you feel it’s more “ambiance fill” sound with the rear speakers, or just more cluttered sound going on. It’s weird at first b/c it’s not what you’re used to and your brain says this is wrong and sounds awful, but then it’ll start to make sense most of the time.
I’m a big 2ch hi-fi guy though, so that’s where some of my views come from vs the whole 5.1/7.1 surround systems etc. My mentality, screw everyone else, I’d rather have it setup perfect for me as they probably don’t care enough to notice anyhow!