Vehicles: Ways to install a drive-by-wire system on any car?

What exactly do you mean by a 'drive by wire' system for your car? The mobility systems you refer to are not 'drive by wire', they're systems that convert whatever input the disabled driver is able to make into something that can be used to control a vehicle, usually by providing powered assistance to reduce the effort needed.  The driver still remains in control of the vehicle! If you want a true 'drive by wire' system, consider this: Many cars already have electronic throttle systems, so the gas/accelerator pedal is connected to the engine via. a wire that talks to an ECU... You could (in theory, depends on your skills!)  build electrically operated transmission selectors, (or clutch and gear selectors) plus electrically operated brakes.  (In that the braking effort you apply to a pedal or whatever is interpreted by a computer system that then applies pressure via solenoids etc. to a conventional hydraulic system.  So, pretty much like a modern ABS system when it kicks in). Steering systems will be fun...   You're still going to need something to actuate the steering box/rack, a stepper motor of some kind, or hydraulic rams could work, (as in full sized cars that are converted to work by RC., by Mythbusters etc..).  You'd want some kind of feedback sent back to your control unit, otherwise the car will be undrivable. Then connect all this up to a computer and, with feedback from steering & braking systems, you could, in theory, connect it all to  joystick type sidestick control.  Develop your software & the computer could take over.  All you'd need to do to achieve the latter is develop a navigation system accurate to a few centimetres, a system to recognise & react to the movement of any object in the vehicles path, lane recognition systems, hazard avoidance systems, AI systems that can handle the dissemination & interpretation of MASSIVE data sets...  etc., etc., etc... Google etc., are currently spending countless millions developing this technology for the long-term.   The basic stop/go/steer issues aren't the problem.  Everything else is. Minor details like the shear illegality of what you propose will be the least of your problems....

Beware of the law! You cannot do this just anywhere if you intend to drive on public highways. There are a heap of "standards" and "industrial" standards to respect. Look it up before going full prototype on the road.  These kind of mods are expensive for a reason. Also, normally, your insurance should be notified if  you are going to mod  a vehicle to this extent