I miss doing longform project writeups here; it's been a while! I recently replaced the old stock stereo in my 2009 car with a modern system (nothing fancy, just a super basic bluetooth stereo because I was tired of not having the AUX cable adapter for my phone since phone manufacturers decided to hamstring the world's phone users by removing AUX jacks).
It came out surprisingly affordable to upgrade, at $62 total! After-tax cost breakdowns:
DIN dash trim (Metra brand): $20
Radio wiring harness (also Metra): $10
Antenna wiring harness (optional): $9
Replacing an older stereo is theoretically straightforward, but annoying in practice because you have no idea what you need until you actually open up the car and realize you were supposed to buy another simple part they didn't tell you about. I got mad and wrote this post up because I had already yanked the old stereo out of my car before realizing I was supposed to buy a wiring harness, and then my car dash had a big hole in it for a few days while I waited for it to ship.
What you need to wire up an aftermarket stereo:
The stereo kit, which should come with plugs that are compatible with the stereo itself
"Dash trim kit": the molded plastic piece on your dashboard that components are mounted into. It should be made for your particular car. This is only necessary if your new stereo mount is different from your old stereo mount.
"Radio wiring harness" (connects stereo to power & speakers) and "antenna wiring harness" (if you also want radio reception). These are sockets matching the proprietary connectors for the wires coming out of your car.I had to buy mine online because my car is 14 years old. If your car is newer then you may be able to find the parts at a local auto store.
Either soldering equipment (soldering iron, solder, electrical tape, wire stripper, and "helping hands"), or you pay someone else to install it for you, in which case you probably aren't reading this post lol
(My new stereo conforms to the "single DIN" dimension standard. DIN sizing is blessedly standardized across cars. Single DIN is the smaller size that lets you install a little snack compartment or drawer beneath your stereo, and double DIN is the bigger size that's usually used for a touchscreen.)
You probably can't just buy a single cable that will connect your new stereo directly into your car, for many reasons. Car manufacturers are based all over the world, and cars in general are extremely non-standardized. Different cars and stereos all use different plugs, and it would be too difficult for each stereo manufacturer to provide a different wiring harness for every possible car model out there.
Also, it is beneficial for every electrical connector in your dash region to be shaped differently, so that you can't accidentally plug it into the wrong socket. At first I was annoyed to see five totally different proprietary connectors when I opened up the dash, but after failing to label three of them, I was very glad that they were all different shapes.
So unless you wanna sit in your car soldering your stereo directly to your car with no takebacks, you probably want to make yourself that custom stereo<>car cable by getting the radio wiring harness & antenna wiring harness that fit your car model, then soldering them to the cable that came with your stereo kit. Thankfully my cables seemed to come with a standardized coloring scheme for the wires, so it was easy to match them up.
Tools you need to replace your old dash & stereo (I winged it with whatever was in my toolbox...but most people have fewer tools than me)
Either an auto trim remover kit, OR two small pry bars, a flathead screwdriver, and something long and flat such as a butter knife for leverage
Philips screwdriver -- a ratcheting one is helpful, but not required
Any additional tools as recommended by your car's service manual (or friendly local Youtube). For example, my car's service manual called for an 8mm socket wrench with a 6" extender to remove a bolt securing the stock radio.
Having two mini pry bars was helpful for taking off the retaining clips without breaking them. They are very fiddly.
Also, my stereo felt slightly wobbly in its thin sheet metal mount. So I stuffed a bit of cardboard between the stereo and the snack compartment as a makeshift shim, and am just crossing my fingers that something so close to the back of the engine won't catch on fire. Maybe the proximity to the AC helps.