Say I have a Alpine deck that was installed by a shop with the kit and what not, and wanted to switch it out for a Sony, would I be able to use the same kit and cables to just "plug and play" or would I have to get something specific for that Sony to install it?

you would have to un splice the harness from the alpine harness to the adapter, and then splice in the sony harness to your adapter.

Either way, you can use the wiring adapter again, just not the harness off the deck.

Maybe that way will be easier to understand.....LOL

for $14 or so just get another connector harness and use it with the new sony deck

Yup. Another kit of the same manufacturer to match the kit you have, and you can plug/play all you want.