"If someone tries to get into my car to attack me, when is lethal force justified?"

This column cannot provide legal advice; and in the situation you describe, no lawyer could give you timely and useful advice anyway.  This is a very short and incomplete response to the question, which takes a solid hour to get through even the basics.  Here is how the situation is typically analyzed:

A.    The clearest answer is, "when you are parked in your own attached garage".  Colorado allows a claim of defense of your own home as an absolute defense, for acts against an aggressor within an attached garage.  This does not apply to detached garages or carports.  The defense however, under the Defense Against Intruders, allows you to dismiss a prosecution before it really begins.  The defense requires you can argue the following:

1) Intruder has made an unlawful entry into the dwelling, and

2) when the occupant has a reasonable belief that such other person has committed a crime in the dwelling in addition to the uninvited entry, or is committing or intends to commit a crime against a person or property in addition to the uninvited entry, and

3) when the occupant reasonably believes that such other person might use any physical force, no matter how slight, against any occupant.

B.    The next part of the answer relies on a couple analytical tools we will use repeatedly in these situations. 

1.     How is the attack being carried out?

A lethal defense against a deadly weapon is justified in most instances, when that deadly weapon is employed against you.  This is limited however to true deadly weapons like guns and knives, as well as simulated guns, for example.  You would have a reasonable belief that even a simulated gun is a deadly weapon, and could respond with a similar level of force.  A club or stick with a nail can be a deadly weapon, depending on how it's used.  There is no absolute defense, however; you could still be arrested, investigated, put on trial, and forced to defend against a criminal case. 

You are then put in a situation where you have to decide whether you can argue a reasonable fear of the deadly weapon, and how it was employed.  You must be able to argue that no lesser degree of force was adequate.  You can argue it was in response to what you believe to be a robbery, under the self-defense statute for deadly physical force.

If instead you have someone attacking you without a deadly weapon, then we are down to fists and feet, usually.   Certainly a foot, or a fist, can become a deadly weapon; and the courts have held this is so, repeatedly, but again, depending on how they are used.  You cannot respond to a punch in the nose, with a gun.  We have a scale of uses of force by an attacker, and you can't 'up the ante' on that scale without risking consequences.  The same thing applies to kicking someone on the ground; you cannot over react to the use of force by turning the tables on your attacker and potentially killing them.  Your use of force must be proportional, or match, the attacker.  There is also some language in one case that allows you to argue a stronger response to multiple attackers.  Bottom line however is no guns in a fistfight.

                Obviously this assumes you are not being beaten to death; or kicked when you're down.  In a fight like that, there is no way to tell in advance that the attacker is going that far.  This is a primary problem with the expression of self-defense in current law, and there is no way around it; you can't shoot in a fist fight, then argue how severe the beating could  have been, without some prior basis to believe the attack was deadly.  This leads to my next point:

2.    Is there any clear imminent threat to anyone else, or a direct threat to your life or a threat of imminent serious bodily injury?  How much can you justify based on what you know?

You may be able to react to even an ordinary use of force if there is a deadly threat in combination with a manual use of force, like fist or feet.  This requires some expression of intent by the attacker, or some way to corroborate your belief.  You could also use deadly force if, for example the baby is in the back seat.  You would not allow a carjacker to pose a threat to a third party, particularly a child or vulnerable adult, without a response.  The attack would constitute a kidnapping, and would allow you to argue defense of others in a situation that was inherently risky to the third party.  The same doesn't apply to the family dog, who is on his or her own.

You cannot otherwise use physical force to stop an unarmed carjacker.  Let the car go, and call police.

You also cannot use force if you provoke the unlawful use of force; or you are initially the aggressor, which is difficult to determine in road rage situations. 

3.    Do NOT use REDDIT as a source of advice.  I can't stress this enough, but I have seen bad, horrid, and then just face-palm-awful advice on REDDIT by folks I assume are not lawyers, and have not talked to lawyers.  Whatever you do, avoid REDDIT for legal advice or guidance.