Disclaimer: I have very little electrical knowledge.
I have wired electronics (radar detectors, dash cameras, GPS units, etc.) to about a dozen vehicles in the past, but have ran into a strange issue with my 2016 Toyota Tacoma (which, I'm told, uses Can Bus).
I've found a switched 12V source (sensor on brake fluid reservoir); verified using a multimeter. 12v when the truck is running, 0 when it's off. I've spliced in a 12V accessory, and at this point the voltage dropped to 5V. If I disconnect my accessory's ground, the voltage goes back up to 12V.
I have tried numerous grounding locations for my accessory - does not seem to make a difference.
This appears to be the only easily-accessible, switched 12V source where I'm looking (engine bay), and I'd rather not run cables through the firewall to an in-cabin source.
What do I need to do in order to have my accessory run off this 12V source?
I'm guessing I could wire my accessory to the truck's battery via a relay, and use the switched 12V to control the relay, but I've never done that, so it's just a guess. I would prefer to avoid such complications, if possible.