the simplest way to check the black wire for ground is connect a test light from battery live to the black wire. if it lights then yes it goes to earth. that's how i'd check it anyway.
neil.
Thats necessarily accurate. Ohms law says that voltage will drop across the total resistive load and proportionally to the number of load and the ratio of resistance between each. So if I have two 8 ohm loads in series such as cluster lamps, each lamp will drop 6v (half the total incoming voltage assuming its 12v). Before someone tells me, this is precisly why automotive loads are not wired in series. But adding a test light will add a second resistive load in series to what ever is prior to the test point of the wire being checked. If I had a 8 and 4 ohm load, the 8 ohm load would have a voltage drop of 8v and the 4 ohm load would drop 4v.
So adding the test light to the black wire found with the possibility that it could be a ground, could result in it being lit (even dimmed) as there is resistive load and as such would create a voltage drop.
Normally, I would check by voltage with a meter. But given the state of the current harness, I can not trust that 12 volts is even getting to a load and going to ground thru that wire. I can not perform a continuity test as that assumes that this ground goes to a junction and is grounded else where. I am really trying to NOT cut the wrapping around the wire to track where its source is.
The best option is to find another example and see if the 3rd wire bundled with the 2 brake light switch wires goes to ground, if not then where.