71 and back I wouldn't even attempt to guess, but on a 72 or later, 10 to 1 it's a bad ignition switch.
The replacement switches on the market today have a life expectancy of about a year . . . but a lot of 'em die young. When you buy one, you might as well buy two and keep a spare in your glove box. If you've got a spare, you can plug it in and try it to make sure that's the problem before you change it.
There not hard to change. After you've done it a few times, you'll get real good at it. You need a small flat screwdriver, a 6mm allen wrench, a 27mm (or is it a 24mm?) deep well socket and a ratchet.
Take off the horn button (snap it out, twist it 90 degrees, disconnect the wire and lay it aside. Take off the big nut that holds the steering wheel and pull the wheel off. Under the bottom/back of the collumn, unplug the wires going to the switch (one big plug), the wires to the turn signals and wiper swithes (two thin plugs that snap together) and unhook the windshield washer hoses (if they were hooked up). Remove the four little screws that hold the wiper/washer and turn signal switches in. Pull the two switches out as a unit. If they are originals and you take them apart, chances are good that the plastic around the pins that hold them together is cracked and parts might fly out of them. Now look in the hole under the plastic cover piece and take out the 6mm allen bolt. Slide the whole lock assembly housing off the end of the collumn. One screw holds the switch in. Change it out and put it back together in the reverse order.
Oh, yeah. There is a little plastic bushing inside the bearing that won't be intact anymore. I always just wrap electrical tape around the collumn to take up the slack and let it go at that. The tape will probably outlast the new switch you put in. :?
