Never tried it, but this function should do the trick:
fnc_enablePersonTurrets = {
{
_role = assignedVehicleRole _x;
if ((_role select 0) == "Cargo") then {
if (count _role > 1) then {
_this enablePersonTurret [_role select 1, true];
};
};
} forEach crew _this;
};
use:
yourVehicle call fnc_enablePersonTurrets;
For this, the suitable positions in the vehicle have to be occupied first.