Ferg

I had a Mk1 octavia that had the same problem. I couldn’t find the leak. I eventually removed the passenger seat and removed the sound deadening under carpet and cut it out to dry. When it was peeled back I looked for ingress when it was raining and parked up but no joy. I wventually noticed it had built up a pool of water after I had the air con on. I noticed a tiny drip from around the housing of the air con unit housing where it fits to the bulkhead ( above passengers right foot). It appears that the condensate from the air con drips onto this seal in the engine bay. If there’s a crack in it, it’ll seep into the footwell. I siliconed up the seal in the engine bay and that seemed to make the problem better. I also removed the rubber bung in the passenger floor. I figured it would drain more water than allow ingress. That worked for me.

I had the same problem. I found out that the air conditioning unit, which is mounted to the bulkhead above the passenger footwell, wasn’t a sealed correctly. So the condensate outlet (peel back the sound deadening on the engine side of the bulkhead, there’s a flap cut out for it) would drip constantly on this joint and leak into the footwell. However it tracks back to the rear passenger footwell, especially if it’s parked facing uphill, like my driveway. There was also a dent in the floor of my front passenger footwell next to the drain plug. I took the plug out and and sealed the engine side of the air con unit with silicone and it seemed to make things a lot better. It might not be your problem but it looked like mine had been leaking slowly for a long long time.