New car stereo wiring question

The fuse box would be the best spot to connect to. My fuse box had empty 1/4”spade piggyback connections, yours probably does as well.

I agree. I installed a vintage 8 track/FM radio in my Duster 340 and connected to the hot battery lead at the fuse box. The player/radio has its own in-line fuse. With 8 tracks, you need constant 12v (not "key on") or the player may be power starved at start up,etc. and damage can occur to unit or tape. Also, if connected to "key on" power, other things are demanding power when you're just listening to the stereo without the engine running. This can result in overheating wires at the bulkhead.