why not use the motor to drive the vehicle and the piston to actuate the steering? Remember the KISS principle it is your friend despite how much you hate it.
anyways if you insist on using the piston to generate rotary motion you can tie the control valve to the rotational position to control what position the piston moves for that angular position, i.e. the piston moves up 0-180 and down 180-360. if your piston only a power stroke in one direction you would need to start in the region of that power stroke, i.e. for a up only power stroke it would need to start in the 0-180 range. if your piston is powered in both directions it could start in either the 0-180 or 180-360 regions but if it started at the top dead center TDC or bottom dead center BDC there would be no torque to rotate the. the possible starting regions will actually be a little smaller because near the TDC and BDC positions the torque would be too low to overcome friction. I suggest you take a look at some steam engines.