Have you got access to a signal generator, by any chance? I ask because a car is electrically very noisy, and the last time I installed an Arduino in a vehicle, crashing was a major problem ('scuse the pun).
Apart from that, a variable frequency, variable m/s ratio square wave generator is what you need, and if you don't have one, kits are available very cheaply. Testing your device in your workshop (rather than in the car) has two major advantages: first, it would show whether it's a problem with the Arduino or due to the electrically noisy environment; second, debugging is vastly easier on your workbench.