I had one until just recently. The problem with aftermarket vs factory is in the windshield. The one I had (can't remember the brand name) had a very accurate reading, auto dimming, and even an alarm that would beep at set intervals to keep you awake. The problem was ghosting on the laminated glass. Always got a double image. It came with a stick on screen for the glass but it looked pretty awful from outside the truck. I made my own screen that was black, small and stuck to the glass with small Velcro disks. Problem was when I parked in the heat, it would slide down the glass and leave "snail trails"! I finally tossed it. It was only like $30 off Amazon. Plugged into the OBD port. Cool idea but too much hassle for me.
If you look through my build thread (not sure which page) you will see pictures of it installed and the screen I made.
​​​​​​​Mike