Some 240s have the amps, and some (like mine) don't. It's a very simple job if your 240 doesn't have the amps in the doors, because you can just observe on the oem speaker which wire is + and which wire is -. Then make/buy a 4x6->6.5 adapter plate, drop in your new speaker and wire it up accordingly.
Here are some pictures of when I put in my 6.5s up front. I didn't have to do any wiring or stock-amp-bypassing at all. Just clipped off the connectors for the OEM speakers, crimped on some spade connectors and dropped in the new speakers. GL with your project
The black shit in behind the plexiglass is some kind of foam tape. It helps the plexi seal against the metal door better, and it helps reduce the plexi resonating/vibrating.