Looking to steer my car using xbox 360 triggers.

Try something like this. (not tested. I use something similar, just stripped out my custom stuff) :

 void Update()
 {
      var rightStick = new Vector2(Mathf.Clamp(Input.GetAxis("Vertical"), -1, 1), Mathf.Clamp(Input.GetAxis("Horizontal"), -1, 1));
       
       Vector3 dir = transform.forward + (transform.right * rightStick.y);
       Quaternion rot = transform.rotation;
       Quaternion toTarget = Quaternion.LookRotation(dir);
 
       rot = Quaternion.Slerp(rot, toTarget, Time.deltaTime * 2);
       Vector3 euler = rot.eulerAngles;
       euler.z = 0;
       euler.x = 0;
 
       rot = Quaternion.Euler(euler);
       transform.rotation = rot;
 
       transform.position = transform.position + (transform.forward * rightStick.x * _speed);
 }