Ok I know this may not help but this was what happened with my truck (which I thought was odd but it was similar to this I think, but here goes...this may not help but it could give another idea...

when my truck lights didnt work correctly I too thought, switch and wires and that mess, well I checked the parts they were fine so I just on the off hand figured what the heck, check the lights, so I looked and my bulbs were 3056 not the 3157 as they should be, granted they are LEDs I have but my driver side would act as if the breaks were pressed and the passwenger side would only flash (till it shorted out) no stop light, so I swapped out the bulbs and all is working well...

don't think this will be what your experiencing but figured id throw it out just incase as this is what i experienced...