My car is an old (2001) Volvo V70.

Last year I experienced intermittent fails of brake lights, and it led my mechanic to exchange the break pedal switch: then all seemed to work fine during some months.

But now my brake lights fail again, and this time it is always.
Obviously we checked all the involved fuses: the issue is not here.

Now my mechanic suspects it might (or "should", he says) come from the big black box located in the trunk, near the rear left wheel: it contains a number of fuses but also a lot of electronic components (relays?), which are not commented at all in the vehicle documentation.

Is there some way to identify which of these components might be responsible of the brake lights fail?

My car is an old (2001) Volvo V70.

Last year I experienced intermittent failures of the brake lights, and it led my mechanic to exchange the break pedal switch. This seemed to work fine for some months.

But, now my brake lights have completely failed.
Obviously we checked all the involved fuses and the issue is not here.

Now my mechanic suspects it might (or "should", he says) come from the big black box located in the trunk, near the rear left wheel. It contains a number of fuses, but also a lot of electronic components (relays?) which are not described at all in the vehicle documentation.

Is there some way to identify which of these components might be responsible for the brake lights failing?