Just as an update, I've decided to go with a different keypad. You see, the one pictured above has a solid membrane across the front. The first few buttons had a nice tactile response to being pressed, that is, you could 'feel it' when you pressed the button. But the last two, although they did work, didn't feel like you were doing anything. Since I'm ridiculously picky (and worried about longevity) I found another keypad. I found this one on a 2008 Escape: (Although it's on heaps of other models/years)
It has a nice feel, is good enough for Ford to use on their vehicles now, and still has the backlight. Unfortunately, it is not wired the same internally, so my old board is useless. I rewrote my program to handle the new keypad properly, but I still needed a new board. Instead of making another monstrocity like that first board, I learned enough Eagle to design a PCB and I've already sent it off to be profesionally fabbed. So now I'm waiting on this to come in the mail (a couple more weeks to go):
Note: That 1K resistor on the right side, above the 330, is incorrectly labelled. It should be 100. This board should fit just perfect into a radioshack 1x2x3in project box, and already have holes drilled to line up with the internal mounts in it.
I have the weather-proof external connectors, all my headers, resistors, internal connectors, and optical relays also. This board is more modular than the other one with quick disconnects for all external connections, and I'll be able to easily remove the Arduino Nano for reprogramming. I think I'll stick it in the door with some servo tape, and see how it holds up.
I'll post more when I get the pieces in!