Most recent cars already have a speed sensor wire present in the connector for the radio (european cars anyway).
If not, it can usually be tapped into an ABS sensor or into the speedo itself (like old models miata and those without the special audio system).
On my '91 miata I hooked it up at the speedo. On the Mercedes, I tapped into the right front ABS sensor wire.
Even if you have an old car without ABS or with a mechanical speedo, there are little boxes available that you can connect the the speedo cable (at the gearbox).


Frank