That's not what they are for. On GM cars, that pigtail went to the alternator. On the ground cable it tied to the chassis of the car. Sure you can add accessories to it. But why not add them at the fuse panel (unless you don't have a console).
Well....In the manual for the fish finder I got it says, if possible, to hook it up direct to the battery with an inline fuse. Apparently, there is less chance of interference from other electrical stuff?
I have locators all over my boat and none of them have any interference. In fact the transducer for one of them is mounted on the trolling motor and it doesn't have a problem. The shortest route to the locator is the best route and will result in the least chance for interference. It certainly won't hurt, but in my view there is more electrical noise at the back of the boat where the engine lives. Why run power wires along the engine harness all the way to back of the boat. That harness can harbor alternator noise and ignition noise and the tach pickup is in that harness as well. It's an over-stated suggestion in my view and experience.
