How can I access a collider's parent's rigidbody?

I have a vehicle and I’m attempting to implement a speed boost box collider. It seems like I’m accessing the parent correctly, but it’s not working.

I have a game object with a rigidbody attached and the collider is a child game object with a box collider attached. I don’t get any error saying it can’t access the rigidbody, but it’s not doing anything.

Here’s what I have:

using UnityEngine;
using System.Collections;

public class SpeedBoost : MonoBehaviour {
	public float boostForce = 10.0f;
	// Use this for initialization
	void Start () {
	// Update is called once per frame
	void Update () {
	void OnTriggerStay(Collider other) {
		other.transform.parent.rigidbody.AddForce(other.transform.parent.rigidbody.transform.forward * boostForce, ForceMode.Acceleration);

This works as a script attached to the vehicle using the line:

base.rigidbody.AddForce(base.rigidbody.transform.forward * NitroForce, ForceMode.Acceleration);

What am I missing here?

I think you are doing it right. But the AddForce needs to be applied over time, usually in FixedUpdate. Or if you really just want it applied once, you need more force than 10.0
Experiment with different values for boostForce

See Unity - Scripting API: Rigidbody.AddForce