I've done that several times! My first carputer only had a character LCD and occasionally windows popups would take focus away from my app detecting keypad presses. I learned to keep an old 15" monitor in my garage. If you're in good standing with your local Comp Renaissance (or other overpriced used computer parts supplier), you can usually get a deal on one of the hundreds of old monitors they have laying around.
I just saw your sig - Jeeps must like to torment their owners when computers are installed!