Recently, keyless starting has become a popular option on many new cars. So why should you be denied the sublime joy of starting a car without a key just because you happen to drive a two-decade-old piece of crap? You shouldn't, that's why. So I'm here to help, as long as you're willing to accept that keyless starting is just a candy-ass version of hotwiring.
Knowing how to hotwire a car is just one of those things anyone even remotely interested in cars should do; not so you can go out and steal cars (what you read here probably won't really help you with that) but so you can get a better feel for your car as a magnificent, understandable machine. For many readers, I suspect this will be at best a review, but for those not familiar with some of the basics of how cars work, relax, this is easier than you think.
Alright, so let's see what we need here. First off, a car, otherwise this'll get boring fast. Ideally, your car, or a car that no one's going to freak out about when you get it started sans key. Also, the car is most likely going to need to be from the mid 90s or older. By the later 90s and up, pretty much every car came with an immobilizer and alarm-type system, and hotwiring something like that is a much more involved process. The latest cars I've actually done this on have been mid-90s era cars, so let's call that our upper limit.
You'll also need at least three clip leads (you can get them at Radio Shack or wherever), some electrical tape (get that out of your uncle's drawer), a small knife of some kind (X-acto, pocketknife, etc), and possibly a screwdriver.
Oh, and I should probably just make it absolutely clear that this is not a comprehensive guide on how to steal a car. We have video games for that. This is a simple way to start a car without the key, and is really at best a party trick or learning experience, since pretty much anything made after 1969 has a lockable steering wheel, and I'm not even covering how to get past that. So everyone can relax. I did this with little kids in two major cities, and everyone had a blast and there so far have been no Oliver Twist-style crime waves.
Fundamentally, hotwiring a car is about carefully evaluating the engineering of a car's ignition lock, being impressed by it, and then avoiding it altogether. Car ignition locks do their job very well, and there's no point fighting with it. Think Indiana Jones and that fancy sword guy. The lock is like fighting the guy with a sword, and the gun is like going around the lock. We're using the gun.
The trick here is realizing what's going on when you turn the key in the ignition: switches are being closed. That's it, really. Turning the key is basically just connecting three things together: the battery, with its delicious 12 volts of creamy electricity, the ignition/electrical system, and the starter motor. We're just going to connect these three components without going through the lock.
So how do we know how to connect these components? Well, the general answer is via all those wires crammed up under your dash, behind the steering column and under the squirrel carcass that got wedged in there years ago. The more specific answer is that we have to do a bit of research to know what wires we want to use.
You could use a voltmeter to laboriously figure out what wire is what, but I'm going to assume we know what car we're doing this on, and the less time spent contorted on our backs under the dash, the better. So I suggest Googling for wiring diagrams of your particular car (For example, if you're looking for an early Ford Bronco), or going to an auto parts store and looking in one of the service manuals until the clerk tells you this isn't a library, or taking the clerk's advice and going to a library.
For most cars, you should be able to get this information quite easily. What you're looking to find out is the color of the particular wires for the 12V from battery (almost always red), the car's ignition/electrical systems (color varies wildly), and the wire for the starter motor (often yellow, but can vary). Once you have this intel, we can get started.
Okay, let's get to it. Get in the car, and get down under the dash, right under the steering column. You'll likely need to pop off or unscrew or tear out a plastic cover or covers to get to the massive, twisted bundles of wires under there. Once you get the panels off, look at the all the wires. It's overwhelming at first, but you should notice what will most likely be three main bundles of wires: one that snakes to the column-mounted controls on the left (indicators, possibly lights or cruise controls), one that leads to the right-side stalk controls (wipers, erotic seat vibrators, etc.), and one bundle that leads into the steering column itself. This last one is the one we want, as it goes to the ignition lock and switch assembly.
Look in that bundle for your target wire colors: battery(red), ignition/on (whatever color you found out), and starter (maybe yellow, maybe not). These wires connect to the stops on the switch, along with one extra, accessory, which is that first stop that lets you turn on the radio. If you want, you can ID the accessory wire as well, but it's usually not necessary.
Once you identify your wires, give them gentle tugs to separate them a bit from the main bundle of wires. The method I'm about to describe will let you get access to these wires in an easily repairable way— we don't want to actually cut any of the wires, we just want to tap into them. So, you'll want to take your small knife and make a tiny hole in the insulation of the wires, exposing a small bit of the metal wire inside. Do this for all three wires, making sure to make your holes far enough apart from one another so they won't accidentally touch and short everything out. Also, keep an eye out for metal bits of the car that may touch and short the wires as well.
Once you scraped little holes in the insulation, take your clip leads and clip right onto the exposed metal, making good contact between the little metal bald spot and the metal teeth of the clip. Do this for all three wires, so you now have three dangling clip leads hanging down from the dash.
Okay, we're almost there. Now, take the battery clip lead and the ignition/on clip lead and clip them together. If all is good, you'll see the usual dash lights come on and whatever beeps your car likes to make when you turn the ignition on, normally. This is the equivalent of turning the key to the second stop. The car's about ready to start. If it's a manual, you'll likely need to push the clutch in, if it's an auto, it'll need to be in park or neutral. Once you're ready, touch (don't clip) the starter lead to the junction of the other two connected clip leads, and the starter motor should crank, and the engine should belch itself into glorious life. You'll probably get some sparks as you touch the leads, but they should be nothing to worry about, unless your floor mats are made of gasoline-soaked straw.
Once the engine starts, remove the starter lead from the connected pair, and the engine should keep running. To kill the engine, simply unclip the 12V battery lead from the ignition lead. And that's it!
See, easy, right? Once you're done playing around, use the electrical tape to carefully cover up the insulation holes you made in the wires. We don't want any shorts or fires or anything. I really believe this is a good thing for people to try doing. It's easy, informative, brings you closer to your car, and makes you just that much more of a badass because, hey, you've hotwired a car. Oh, and if your key ever breaks off in the lock, it could save a lot of hassle. Enjoy.