Reverse lights when brakes applied

Hey all,
2006 Mazda 3 GS, 156000km, auto.

For quite some time now, when I applied the brakes, the reverse lights will come on as well.

Thinking it was a simple fix, I went to tinker today, and it seems it's purely intermittent.

How can I narrow down what this is, and what todo?

Do you also get brake lights when backing up? Sounds like the wires are crossed- pinched somehow through the insulation. Last time I saw this was after a shop replaced the fuel tank and they did not watch to make sure the conduit to the rear was out of the way and they pinched the entire conduit in behind the gas tank bracket- flattend it out and crossing up the two circuits- through the insulation.

Has any major under carriage work been done?

Another common are may be in the tail light assembly- check that the wires are separated.