How do I locate a short in my Toyota's electrical system?

Usually, you pull the negative battery off and connect a test light or voltmeter between the cable and the battery to check for a large draw and start pulling fuses until the large draw goes away...that indicates which circuit has the short. Most modern vehicles have a small parasitic load so there will be some draw of a few milliamps. Then it's a matter of tracing the wiring for that circuit to look for evidence of the short such as grounding wires or traces of arcing. It sounds like the lighting circuit is the problem...start with the ignition or headlight/dimmer switch and it's ociated wiring/connectors. Check for bad grounds also as they may be causing the current to seek another path. Eautorepair.net has wiring diagrams...small fee.