JFYI, those are not necessarily wired incorrectly, car wirings are "queer" a single ground connection (negative/black to frame/body) that has oxidized or however does not make good contact and anything can happen to lights, and tail lights particularly also often suffer from dispersions/bridges in the connectorts due to the effects of humidity/mist[1].

[1] some makes/models are particularly subject to this (FIAT do you hear me?) , and it is very common to need to spray some WD-40 or similar once a year or so on contacts/connectors of tail lights/turn lights.