Owner replaced RH headlamp with a different year impala, and now while pressing the brakes the lights in the rear would not work. All fuses and bulbs are still good and still does not work. What could it be?
Try disconnecting the new headlamp housing and see what that does. These newer cars are very critical about their electrical wiring. I have heard of one fellow whom put in the wrong bulb for a brake lamp which made his transmission act up until he put the proper bulb in the socket.
If removing the headlamp housing fixes the brake lamps then check and see about getting the proper lamp housing or comparing wiring between the two units to find the culprit in the lamp circuit.