[DCSS]

I agree with stoutfiles views on that a carjacking is normally something that happens to you when you least expect it, in an instant, therefore it's not prudent or possible to pull a weapon yourself and use it. Most occur at intersections or while your getting in or out of your vehicle when it's parked, and often times while you are pumping gas or putting kids in carseats. Most people never see the c/jacker even approaching them...due to not paying attention to their surroundings, on the cell, or just daydreaming etc...just imagine sitting at a long red light, maybe you change the radio station, look at a text message or something to make you look away from your windows and rear view mirrors. Regardless what you do to look away from what's going on around you for that moment, when you do look back you notice a minivan just pulled up besides you and the sidedoor opens, out jumps a man, with a mask, pointing a gun at you through your window (it will look like a cannon)...he's only about 5 feet away...what are you going to do now? Most people will freeze and comply (human nature) and do exactly what they say (you say otherwise maybe your lying to yourself right now, military, off-duty Officer, Clint Eastwood, ex-con, whoever you are), regardless if you have a gun on you, in the glovebox, or console...there is no way you can get yours out and shoot or defend yourself without him seeing it, in that instant (cept tinted windows and most will not try to jack that just for that reason). Even if you get the 'drop' on him he can still return fire, and he is right next to you.
If you do have the chance to arm yourself during this, yes by all means you have a right to defend your life or the life of another...when you can justify it by saying you were in fear of your life or that of another. When you do this and if they die...it's a homicide, but justifiable. When an Officer uses force and death results, they are techinically charged with homicide...they just get cleared of it once it's ruled justified...that's why Officers involved in use of lethal force are placed on leave for so long afterwards...the legal process and investigation start and takes time. They don't just return to work afterwards like in the movies. Citizens have this same right to defend life.
Just be correct and as mentioned in a comment above, I wouldn't advise shooting at your car as it drives away after you get it taken from you...that's a f' you shooting and your not defending anyones life at this point. You just may be wearing a striped jumpsuit afterwards, if you don't care that's on you. Some jurisdictions have laws to protect property also, but it's not that many and I can't comment on that.
Also victims being placed in the trunk during a carjacking is not uncommon. Think about it...if you took someone's car at gunpoint, you want to use that car for some reason. You let the person go and know the first thing they will do is call the police. Now what? You have maybe a little time before onstar, lojack, etc...are activated and every Officer is looking for this vehicle, because they want to get in 'that chase' (can't beat that adrenline rush-skydiving doesn't compare). No you put the person in the trunk and you have all the time in the world to use this car for whatever you needed it for. Cajuntyes had a good idea with a Lojack button in the trunk.
BTW for those living in the DC metro area, especially the DC-PG County border areas, carjackings are very common, almost nightly, they are all not reported on the news. It's quite the opposite, only a few chosen stories are reported in the media. Ones that are from affluent parts of the area. It's probably the same across the country.
The best way, I believe, to defend yourself from this even occuring is to pay attention of your surroundings always. Leave space in between your car and others at lights, plan ahead where and when you will get your gas, etc. By all means, if you can legally arm yourself and you want too, do that also...but remember if you can't get yours out first-it's just a piece of metal sitting there and can do nothing to help you, unless you think bullets will bounce off of you and take that chance to grab yours while one is pointed at you. It's real life folks...they don't care, they just want the car your in. One way or another, in any condition, it will be returned at some point to you.
Be safe everybody and this was just my point of view from 20 years working the streets of SE DC. If it's going to happen to you, it will be a life-changing event and your property isn't worth your life.