It's kind of late to say it now, but using the insulation displacement wiring taps behind a panel where you cannot service them later is not a good idea:
My former service manager was posted to Lake Placid in 1980 to handle problems with radios for security at the XIII Olympic Winter Games. The late '70s Dodges and Plymouths used by police at the time no longer had a spade in the fusebox to supply accessory power like older vehicles did. The 5-amp fuses for the receivers, control units, and transmitter lock-outs were all powered by those blue splices provided by the radio manufacturer, snapped onto a vehicle accessory wire going to or coming from the fusebox. The only failures he had to fix while he was there was hard-soldering the power wires to accessory wires when the blue taps failed. Since I heard that story, I have never used these again.
I would have twisted, soldered and put heat shrink on these wires, the way your stereo was done. That way, I know I could sleep at night. I know you put LED lights on the outside, and I think you said you were going to also use LEDs "so you can see" (inside?). So with the low current of LEDs, the risk of fire from a bad splice is about nil, but it's possible to have the lights become intermittent. With a series of incandescent bulbs strung together, heat in a bad splice could be a more serious issue. I may have seen a semi-melted splice decades ago, but I don't know if I am remembering or imagining it at this point in my life.
Whenever I needed to make a "Y" connection and had to use a crimp connector, I would use a butt splice instead of a tap. I cut the feeder wire, and twisted the new wire with one of the cut ends inserted into one end of the butt. The other cut end goes into the other end. I would usually squeeze the doubled-up end of the butt a little bit to make it somewhat oval, so both wires' insulation would fit inside the plastic butt insulation right up against the splice tube.
My camper built by a maker to remain nameless (Fleetwood) used these taps extensively to roll the units out the door quickly, and I have lots of DC voltage drop problems that I have been hunting down and fixing one by one.
If you have more wiring to do, or if someone else is looking at your project for ideas, I hope to save some possible grief. The advice is free, and is worth at least as much as it cost.