to make use of all the connections you already have, if your car has steering wheel controls, or the option for them, grab a tiny micro, either hook into an analog speed signal to the dash, or use something like an elm chip or a micro with built in can bus to read in the road speed off the can bus, pulse the volume up/down as required using the micro to scale correctly, done,
That or for certain models of radios, if you dig a bit, you may find the feature can be turned on in the developer menus, some of these are documented online,