There's a wiring diagram for your car located
here. Although the starter relay is in the lower left in the diagram, I believe it's actual location in the car is on the rightside of the engine bay near the ignition coil.
In order to shorten testing time when the starter sticks on, using a voltmeter is out of the question. So pulling a wire or two will have to do. Having previously located the starter relay, and when/if the starter sticks, turn the ignition switch off and quickly remove the smallest of the 3 wires from the starter relay. If the starter stops, then the problem could be either the infamous seatbelt interlock system or the ignition switch. If the starter continues to run, disconnect the wire from the top of the cold start solenoid. If the starter stops, the cold start solenoid is backfeeding the starter solenoid. If the starter continues to run, disconnect either one of the two remaining large wires off the starter relay. If the starter stops, the starter relay is sticking. If the starter continues to run, the problem is in the starter itself, most likely the starter solenoid.
This is assuming that all electrical connections are clean and tight, as suggested by the Sheriff, and that the battery is up to snuff, which Ruedi brought up. Since amperage increases as voltage decreases, inadequit voltage at the starter can and will, if only momentarily, partially weld the starter solenoid contacts closed.