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It sounds like the ground is bad. The grounds for all three lights is common. However, there are splices in them. What I suggest first it to inspect the ground specific to the rear brake lights. I attached a pic. The pic shows the ,main ground for them.
Check that for continuity, make sure it isn't loose or damaged, and check for corrosion.
Let me know what you find. I suspect, the added load of the two additional bulbs is causing the ground to fail. They all get power from the same source, so this is my first suspect.
Friday, October 25th, 2019 AT 6:31 PM