The absolute simplest way is to go buy a PC monitor between 19-22", be it CRT or LCD and mount it on a shelf inside your cab.
I don't know if if Defenders have any kind of shelf built in, but even if they don't you can go down to Lowes and have them cut a sheet of plywood into a square shape that will fit your cab. Have them also cut a 2x4 into lengths appropriate to screw into the inside of your cab for your shelf to set on. After that you just set your monitor of choice on the shelf and you're set.
I reiterate, that is the EASIEST solution, but far from the most authentic or versatile. the most versatile would be a pc monitor that can do low, medium, and high resolutions. They are rare, but worth having a look around your local environment for.
Look here;
http://srtv.ca/Boxing_Day_Flyer.htmsomebody was just giving away a mitsubishi diamondscan 20m, which is a 19" PC monitor that can do many resolutions and can connect directly to your pc. You should look for similar deals in your area.