Is it just me, or does in-car entertainment tend to be either very low on the list of priorities or completely ignored when most people start looking for their next vehicle?
If true, there can only be a few reasons for this. Either they don't care, completely forget about it, trust the factory system to be just fine for them, or they are enthusiasts willing to take advantage of the plethora of hardware available on the aftermarket.
For this article we'll concentrate on source (or head) units, and we'll leave the speaker arrangements for another time.
Even up to a decade or more old, plenty of factory head units are amazingly-good, and in cases where they are integrated with other systems or controls it's totally understandable that you may not want to remove it. And that's okay, you still have options.
You can get amps to accept their (higher voltage) speaker output channels, sometimes retrofit features, or just run external devices straight onto an amplifier and bypass the head unit altogether (leaving it to still run things like climate control for example).
Leaving the factory source unit in place and hiding your new system is also fairly common practice in classic vehicles. Maybe do as they do and tuck the head unit in the glove box, or get a 12V amp that will accept an auxiliary input and play MP3s directly off a memory card or stick. Some amps can work directly with Bluetooth as well.
For some cars that came without MP3 capability you can get an MP3 player (with Bluetooth pairing and a hands-free mic as optional) that plugs into the CD 6-stacker's connector at the back of the head unit, and you just select the "CD stacker" to use it.
It is also possible to get a classic car's standard AM radio retrofitted with current functionality like playing MP3s. If this appeals to you then someone in your car club probably already has one so ask around.
If there's no other audio input available an FM transmitter is a very cheap option, but these are quite prone to interference.
In terms of your in-dash replacements there are now more than just the two standard sizes of single-DIN and double-DIN.
If you want a video display you can get a little screen on the front of a single-DIN, a fold-out screen, a normal double-DIN screen, all the way up to a tablet-sized screen that may require an imaginative dash installation.
Other features available include WiFi screen-mirroring to your smart device, or they may actually be an Android device with all the features and apps that can bring.
While many units may still be a DIN standard, the depth a unit reaches back behind the display now varies with some being very shallow. This means you can get rather creative and mount them in places you would have never attempted back in the day.
Helpfully, there are many model-specific fascia surrounds to make a DIN unit look neat, and there are also several vehicle-model-specific aftermarket units (including the Android-based ones). These will have the right shape surround and suitable wiring inputs/outputs, letting you keep or add many features. But do check if you'll sacrifice any current capabilities like speed-sensitive volume, use of the wheel buttons or the little display on the dash.
Whatever source unit you choose, be sure to check its list of features. Note the file formats it can play and if it will receive Aussie AM, FM, and DAB+ if you want that too.
Make sure it has all the inputs you want, for instance a TF memory card, USB, CD, DVD, Aux audio or video inputs or outputs, reverse camera, digital TV receiver, Bluetooth, a microphone for calls, WiFi, a 4G dongle, or an external GPS antenna.
If you plan to run the mid-range speakers (and tweeters) directly off the source unit, look for the RMS Wattage. Peak power means nothing. The RMS figure is the Watts that the internal amp can run continuously. 4x 20W RMS is about the minimum you'd want for sound quality. Also make sure it has enough line outputs (low voltage for RCA leads) to feed your amp or amps with a better-quality signal. These amps will power the sub or subs, plus the speakers if you want them to.
Sam Hollier is an ACM journalist and a motoring fanatic who builds cars in his shed in his spare time.