When they say "plug and play", that usually only means if you purchase the correct wiring harness. If you don't purchase that, then you do have to do some cutting and splicing of wiring.
It's pretty simple, though, the new headunit should come with a wiring diagram for the harness on it. Here's
a how to on replacing a headunit when purchasing the correct wiring harness adapter, which is also listed in the how to.
If you don't want to buy the right harness, you can find wiring schematics for some of the earlier trucks here
. I'd be shocked if the radio wiring has changed, so this will probably tell you what each wire on the truck harness side is so that you can properly connect them to the harness that came with your new headunit.
Alright, after re-reading the original post, I don't think my previous info is likely to be helpful, I thought the Alpine ktp-445u was a new headunit...