How does the digital dash know your speed? Is it reading it from the ECU?
If so, you may have to splice the wire. It is not a very big issue, as this is not a high frequency signal.
Most speedometers are 4,000 pulses/mile. At 100 mph (not that you will ever go this fast
), you will get 400,000 pulses per hour or 111 pulses per second (111 Hz). This is well below the speed where you will have to worry about any transmission dynamics of the wiring (typically around 20 kHz). Correct wire impedances and termination are not necessary at that speed (although I would recommend twisted-pair wire and probably a shield).