The HUD in our van is a small unit which sits flat on the top of the dash a cable goes around the end of the dash and plugs into the diagnostic port just under the dash driver side , it reflects the image onto a semi clear plastic piece that sticks to the inside of the windscreen . You can see the images without taking yours eyes off the road , whilst I have not had a temperature warning as yet I would imagine it is the same sort of warning I get when over revving it gives a sharp beep tone and maybe a temp sign comes up on the screen but not sure as I said . This is the same as ours but a newer version .
LED CAR HUD Head UP Display OBD II 2 Speeding Warning System Fuel Consumption | eBay