I ised an aftermarket kit to add power windows to several cars over the years, in fact I did it last year to my 2006 GMC Canyon. I think I can do it on a Slate too.
I asked Slate about adding an aftermarket radio and they said there would be a kit available to do it.
i was just going to design and 3D print my own kit for adding a radio with apple CarPlay and possibly aftermarket speakers.
And yes I am a Boomer also. 70 years old LOL
There is one on Thingiverse, but not very detailed. I am working on modifying that design and will post it to thingiverse when I get done. I will have both a pickup and SUV Version.
here is the one available now
https://www.thingiverse.com/thing:702182
here is my progress so far