Rant: spent 45 minutes guessing at wiring on a Nissan harness until I bought a $30 power probe
For the first few years on my own I was just backprobing connectors with a multimeter and chasing grounds by eye, which cost me a whole Saturday on a no start Altima in Glendale. I picked up a power probe in January and now I can shoot power and ground straight to a component and know in 30 seconds if the circuit is even alive. What is the one cheap tool that made you feel dumb for not buying it sooner?