Just my humble opinion... I think the LED's are great for headlamps and reverse lamps, but they are too directional for interior lighting in your combine (unless your goal is to illuminate one spot on the floor of the car). With one LED, you won't be able to achieve that warm glow through the windows of your car; you would have to piece together an array of LED's to achieve that effect. The good news is that LED's are quite happy with 9 volts, and your battery will last longer if you stick with one LED. I just don't think you'll be happy with the light output.
Good luck!
Jay
Good luck!
Jay