[Paul Franssen]

Reading Don Plumley's thread on his "dad and son" road trip, my attention was drawn to the on board road trip computer, on a "regular" (that is: very nice) Carrera.
I am curious as to how these things actually hook up to the car. If it's a simple thing to do (?), I'd consider getting one of those. What are some good brands and what they go for?
Obviously, I'd want to consider a metric machine but your replies would be a nice start, thanks in advance!