Honestly, I think you should put down any electronics, and focus first on cleaning the cabinets and getting to know them inside and out. I used to get cabinets, and if the wiring seemed to work, I didn't even really look at it or consider it. In the last year or two, I changed that process to instead begin with fully stripping the cabinet down, making it squeaky clean (even in the parts you can't see), and putting it all back together bit by bit. it takes about 2-4 hours to do a complete cab this way (better with a friend to help) and by the end you'll have really high confidence of what's in the cabinet and how it's supposed to fit together.
Plus, it's really nice to do anything to the cab in the future, and not feel like you need to take a deep soak bath afterwards from having old cabinet crud on your hands!
tl;dr take it all the way apart and do a deep clean.
Also, Aero pro-tip: by now the rubber grommets for the front glass have surely become a goopy mess and are gone, so it would be good to find something new there. You don't want the screws going on the bare glass, for it will scratch it.