Weep holes missing

[Robbox]

Our old stone house has a recent extension, which is cavity construction. One wall faces SW and gets hammered by the prevailing weather. We arrived several years after the extension built and all that time one window on the exposed wall has damp on the reveals inside.
I have been at a loss to understand what is happening. But now I am certain it's because there are no weep holes above the lintel and penetrating rain is finding nowhere to go but over the cavity tray and into the reveals.
So what is the solution?
The stonework is irregular, to look like a rubble wall. I am thinking to hack out enough stones to see inside what is happening. Then fit the plastic type weep holes.
If the extension is about 15 years old, should I find stop ends on the tray? Can they be reinstated if not there?
Finally, I don't quite understand how the usual plastic weep holes work to remove any moisture.
Thanks.

 

[kbdiy]

If the reveals are damp then my guess would be that the cavity is bridged by brick/block/stone where it is closed around the reveals. In modern builds plastic or similar cavity closers are used to prevent damp bridging from the outer to the inner leaf. It sounds to me that this extension was built using the same stone etc across the cavity.

 

[Robbox]

Without knocking off the old plaster, I'd say yes same stone as on outer skin. They seemed to have filled the cavity with rock wool and that could add to our woes.
I don't know when modern methods of bridging the cavity were introduced, but I have a feeling this builder was not the best.....
When we were repointing the wall, we took a stone out lower down and the wool was quite wet. So I am thinking that after much rain and wind, moisture is penetrating and going into the wool.
I have applied some weather seal stuff to this whole area.