Constant water puddling behind passenger boot rear panel

[BigiusDickius]

Have posted about this over the last fortnight but I'm genuinely stuck to work out how water is getting in and collecting.

Rear inside panel and carpet removed and drying, puddle mopped up but it'll be back.

Rear wash hose goes there but it hasn't been used for weeks and the bits of the hose I can check/see/feel are all secure and no leaking.

Rear light cluster power cable and hole (to plug power into cluster) are dry and cable is also dry...so I don't think it is coming from the back of the light cluster.

Anyone else suffered this and found the cause?

Electrics are all working but not convinced they'll remain working if the water continues.

Any help or advice would be appreciated. Car in for a service next Friday and I'd like to point out potential causes to reduce the investigatory costs.

Thanks.