I have a volume knob on my setup. It's a remote knob for the Matrix Audio Controller. It's in the upper right corner of my dash. The audio controller lives in the glove box. There isn't enough room behind the dash for it.
Oh yea, I forgot you had the one with remote, that is a better option. And since you aren't doing these anymore I guess the three switch swap is just a suggestion for anyone else attempting this. After looking at different options I think the tablet is the easiest. Carputer/Pi/BB all have more potential with complex programming and fun little tricks built, but for simplicity, this is it.