Why under the seat:

I wanted to be able to get to the device easily for a number of reason, including: 1) In case I had the dip switches set wrong, 2) to use the audio-in jack, and 3) in case the device went “snap, crackle, pop” and I had to replace it.

And.. frankly, I tried a couple of locations in the dash, and none of them were easy to get to and none had an easy mounting service. I didn’t want to use velcro or some such. So, after looking at the options, it just seemed to tuck nicely under the seat rail.

Why the lighter instead of a positive wire on the radio:

Just laziness I suppose. I didn’t want to wait to order an OEM wiring harness, and I didn’t want to ruin everything by accidently severing the wrong wire on the radio’s connections. The lighter was easy to reach, the wire was easy to strip and solder, and it was easy to tell the positive wire from the ground. And.. perhaps more importantly, I could do the solder work AFTER I had reattached the radio, and stuffed most of the stuff back into the dash — it made it a lot easier to work on without burning my fingers and/or dripping solder on the center console.

Why did you choose to have the device under the passenger seat and not hidden somewhere under the dash?
Why not connecting the power to the plus on the radio instead to the lighter?