If you do a Google search fro RV Trailer wiring, you'll get several sites that will provide you with the STANDARDIZED wiring for trailer plugs, from 4 wire flat plugs to 7 wire round plugs. You need to wire your plug to match this. The important ones to match are left turn, right turn, running and ground. These four wires will go to specific spots on the plug. The plugs usually have a 12volt charge line nd electric brake position.
Make your wire correct and then find the dash light. Many of them are wired so that power (12 volts) is sent through the plug to power your dash light. You can check for this by using a volt meter at the toad plug. Have someone step on the toad brake while checking the toad plug to see which pin has power.