I used a Radio Shack paddle SPDT switch (model 275-648). SPDT means "single pole double throw". This specification is the same as the automatic squat switch Pete (OP) specifies for the auto anytime switch. It's just of the manually-selectable variety. I found out about this switch here on TS (from wileetundra or Chiplee I think).
If you want to tie into the overhead monitor you'll have to drop 2 wires down to this switch matching Pete's schematic. The 3rd wire to the switch can come from the cigarette lighter for +12v switched power so that's why you would only need 2 to the overhead area. I oriented the switch so that when it's in the down/off position the monitor strictly works as normal (i.e. in Reverse gear). When the switch is flipped on (up) the monitor comes on no matter the gear selector position. Again Pete's design is what I'm using and he's done a great job of describing the wire connections above with the exception that my switch is manual in the panel.
So here's how I modifed one of my panel blanks (on the A/C control panel) to take this switch.
First I took the panel blank and covered it in masking tape and drew diagonals to find the exact center. I used a spring-loaded punch to mark the center:
Then I removed the layout tape and used a step drill bit to open the hole up to a diameter right at the diameter of the switch. A step drill is perfect for this. This is a view before cleaning up the plastic shavings and after Dremeling out the center part of the back of the switch blank for clearance:
I test fit the switch and the hole was just enough of a tight fit I only had to thread the switch into the hole and not use the mounting nut that came with the switch. After removing the switch I soldered three wires to the three switch terminals as shown:
After soldering wires on the switch terminals I screwed the switch into the drilled blank and oriented properly. I then slathered some Permatex Right Stuff gasket maker around the threads of the switch to hold everything in place and allowed it to cure. Right Stuff gasket maker can be found at your local Autozone. Right stuff isn't like regular-old RTV or silicons. It may seem a little pricey but trust me you'll get your money's worth. It's fabulous stuff and we're addicted to it around here.
After the Right Stuff was cured I filled the balance of the switch blank cavity with more Right Stuff to ensure good strain relief and vibration control. The switch leads were soldered to a quick disconnect plug (computer 3-pin fan power plug) with the male on the switch side and the female on the truck wire side:
This is a shot of the test fitting. I also used this same switch for a wileetundra speed pulse generator for my new factory Nav unit. The Radio Shack switches match the panel color perfectly and even have a slight curved bezel so that they sit flat against the face of the modified knockout blanks.