Sideloading apks is a way to get apps onto your android headunit without using the play store
Install unknown apps
Android has a level of self protection from rogue apps and any app not installed via the play store is considered a potentially rogue app (by google). Android OS won’t allow the installation of apps from anywhere other than play, without a little bit of work by the user. This is when you need to sideload.
Found the .apk, what next?
So you have found the .apk you want to install, maybe you downloaded it from your android headunit and it is saved on the internal memory, then that is good, keep it there. If you used your laptop or tablet to download the very special, yet potentially dangerous .apk, transfer it to a USB stick/Thumbdrive or even an SD or microSD card. Pick whichever your android headunit has.
Catch 22
You may find yourself in a ‘catch 22’ situation where you need to sideload an .apk file because you can’t access play store, but you need an apk installer to complete the task. This is unfortunate. Persevere and check each file manager for the ability to install .apk files. You may find your installed web browser can install .apk files.
Allow from this source
Allow from this source
Your phone and personal data are more vulnerable to attack by unknown apps. By installing apps from this source, you agree that you are responsible for any damage to your phone or loss of data that may result from their use.

This message is google making sure the user knows they could be installing a rogue app. It is the message you are looking for. If your file manager, app installer or browser pops this message up when you try to access the .apk , it’s good news.
Use the slider to enable the feature and allow the app to install the .apk. Then go back and tap to install the .apk again, this time you won’t get challenged and the .apk will sideload into your android headunit as if it came from the play store.
You can now use the app. If it installs an icon on the home screen, it should be there. If it is a rogue app, then your problems have only just started. Hopefully you picked up the .apk from a respectable android market and everything is good.