I recently found damp & mould in my boot along the near side and wheel arch trim and the spare wheel cover. 
 

After taking out a load of trim from the boot I found that the leak was coming in through my C pillar but from several different places, the air pressure vent behind the bonnet was allowing water ingress into the C pillar well which was splashing into the boot and there was a gap in the joining seal that Ford applied which also allowed water to flow into the boot, both of which have now been fixed.

 

There’s a 3rd leak however that I cannot find there is a slow drip coming down the front inside of the C pillar. I’ve sealed up all of the light fixings that come though the body work for instance the loom seal and the plugs for the screws aswell as sealing the near side boot hinge and any potential openings where that panels have been joined but the leak it still happening.

Yes I Know the car is old coming up to 14 years in fact but I’ve kept it in really good nick, throughout the last 2 months in lockdown I’ve been working on the car to revamp it and mechanically it’s runs perfectly too so I’m not going to get rid of it.

Has anyone experienced this and fixed it or have any other ideas maybe how to fix it? 

If it's all sealed up from outside elements maybe water leak from rear washer hose? 

Usually there is some tape under the rear bumper under the light. That tape is useless and needs to be refreshed. I literally did it yesterday on mine.