I have considered using some old component sets myself, so here goes imho, fwiw..
Acoustically, car speakers may work well in such an application as most are designed to work flush mounted in a panel, and work well in free air and/or large volume enclosures which may be the case in a large wall cavity or attic. A call to the manufacturer can determine the optimum volume if you build an enclosure.
They are designed to be resistant to heat and moisture, and many have mounting systems, at least for the tweeter that allow it to be aimed.
Hey, this is how most of them are demo'ed to be sold.
The quality of components may well be better than many low-cost in-walls.
May even have a usable grille.
The down side, and its a pretty big one, is most all car speakers are 4 ohm, and very few receivers will tolerate this low of an impedance load, without going into protection, overheating, or distorting. Most all receiver manufacturers give warnings about this and few recommend lower than 6 ohms. If you are using some other amplification you might be alright.
If you already own the speakers, it may be worth a try. Nothing a little drywall and spackling won't take care of if it doesn't.