RomanticMoments

Right, but on vehicles like a Caravan that have multiple speaker locations based on trim level, those databases will show "possible" speaker locations, and they are by no means 100% accurate. The OP has already mentioned there are speakers in the dash, and there are no rear speaks. It's not clear if it wasn't equipped, or if the factory speakers were removed. With a 79, I wouldn't trust any computer database, as most of it is speculated at best. In this case, unless there is someone here very familiar with this vehicle, I would pull the original speakers and see what they are. Since the OP is only really looking for rears, I'm thinking it may be down to simply measuring the hole and figuring out what will fit. Also, I remember doing dash speakers in a Dodge from the same era, and even though we had 3.5" speakers (JBL GTO series) the basket and magnet were too large to fit the factory hole, so this is another aspect to consider. The tunes muscle cars were designed to play came from the tailpipes and tires, not the radio. As for a suitable head unit, there are many out there. OP, if you simply want some sound in your ride, buy a cheap deck from a big box store. If you have specific needs, like iPod, Time Alignment, crossovers (most have at least a low frequency crossover, which is really all you would need) then you will need to purchase a head unit to suit. My thoughts, run a simple system. Focus on a decent set of 6x9 and a head unit that does what you want it to do, nothing more. If you want to make the hot rod sound really nice, you would want to look at some custom kick panels or door treatment to run a nice set of 6.5" components, or even a three way system if you wanna kick it up a notch, amped with a small sub. To me, it doesn't sound like that's what you're looking for, but who knows.

Dukk

I put my fair share of speakers into these things back in the day. If you want to do it as painless as possible, replace the front 3.5s with a set of Pioneers (the tweeter has to be flush with the top of the speaker - it can't stick out) and a set of any decent 6x9s in the back.

Cutting the dash for a modern headunit is the only real question as it's a permanent alteration. If you believe your car to be collectible, you may want other options..