The center brake light works OK, but the brake lights in the tail lights do not. The bulbs checked out OK. There's no power to the tail light brake lights according to my trusty Radio Shack multimeter.

There's two switches on the brake pedal. One is connected to at least the center brake light. When I disconnected it the center light didn't work (and neither did the tail light brake lights.) The center light worked OK when I reconnected the switch (tail light ones still are broke). I don't know what the other switch does. Both wires connected to it are grounded (trusty multimeter again). Is this an indicator of the root problem?

Here's the stuff you need to get at the root of your issue.

The second switch is called the Automatic Speed Control Device(ASCD) and it connects to a small module located on the drivers just under the dash. I doubt this item is your problem since your center light works. Based on the wiring schematic I posted it could be a bad ground on the tail light bulbs. Do you have any corosion in the bulb sockets?

Does your X have a trailer connector?Sometimes the connection corrodes.

I am having the same issue. Did you find out what was wrong ?

Everything was working fine in my Xterra. However the seats and carpet were very dirty and hence, I removed my rear seats, front seats, center console and the carpet. Took everything to a pressure washer and cleaned it.

Since, I refitted all the above the two side brake lights are not working. The center stop light works fine. All the bulbs and fuses checked out fine.