xxx2,3,4 on the potentially bad ground. The best clue that you provided us for helping you in this situation is that "nothing has been tampered with" in terms of the lights, bulbs and wiring back there. This leads any experienced DIY type to point you at most likely a bad ground or, as Pigman stated, a goofed up bulb.
Bad grounds have the highest percentage of causes of electrical issues, especially when it comes to lighting, than any other potential cause. Now, this doesn't mean that you can't have a broken wire touching a grounded spot every now and then, or a bad plug with corrosion. There's all kinds of potential causes but, do check the grounds first.
2004 ITASCA HORIZON 36GD, 2011 GMC Sierra 1500 4x4 Toad '18 Honda Africa Twin Adventure Sports DCT
Retired-29.5 yrs, SDFD, Ham - KI6OND
Me, Karla and the Sophie character, (mini Schnauzer)