Hi,
Long time reader, first time poster and all that.

My -99 D2 Td5 automatic has various little niggles, but the most pressing is this:
When it rains, and only when it rains, it will occasionally do the BCU-disco-dance; chime going off, various warning lights come on in the dash, the turn signals stop working, the slowest windshield wiper setting doesn't work, driving lights activating randomly... etc All this with the key in my pocket. In addition, the engine doesn't turn off with the key.

I tried following the procedures lined up in https://www.landyzone.co.uk/land-rover/discovery-td5-wont-shut-off.306469/ and got as far as cross-checking the relays (ok) and the engine does stop when removing F1.

No oil in harness. I've switched out the BCU a few weeks ago for a used replacement, which fixed the previous bout of discolights.

It's not impossible that these are unrelated issues, but the car did the same a few years ago, also during rain. It stood for two years, and the battery ground cable attacher-thing is seized so it's not got a brilliant connection

Water in the internal fuse box.
Rain water from the sunroof got in mine. Mine would not stop when I took out the key

Thanks, I'll see if I can find something wet under the dash. I've sikaflex'd the sunroofs shut from the outside many years ago (roof rack, so no use anyway) so that's probably not where it gets water from..

The BCU seemed dry as well.

The back of the interior fuse box seems completely dry and dusty, though there is some surface rust on the bulkhead behind it. I fitted a new battery and new negative cable and terminal, so that rules out the battery.

EDIT: I stand corrected. When re-mounting the fuse box, water started dripping out the bottom! I think we've found the problem.

I removed the fuse box, tried unsuccessfully to open it, and instead just positively drowned it in WD40 and chucked it back in. After waiting for the BCU to find itself and allow me to start the car, I'm also able to turn it off using the key. So clearly part of the problem is here. The light show on the dash continues, however, also with no key in. Also wipers don't work as expected, no driving lights, no rear power windows, no blinkers...

I did switch out the battery and fitted a new ground cable, just to remove that possibility. Battery shows over 14v when running, so I'd say the alternator is also working as expected...

I'll try and find a replacement fuse box and see if that helps.

Don't do what I've just done.
Get then right part number for it.
Pre and post face-lift seem to be different

It's not hard to open I cleaned mine with isopropyl alcohol but it was still buggered. I could go through the wiring diagrams and change the cables around or what ever is needed.

Yeah, that'll be your problem right there. I cleaned it up using isopropyl and a paint brush, sprayed some electronics protection laquer on it and called it good. No more issues, everything works like a charm. Thanks for pointing me in the right direction, I probably would still be messing with the BCU. Let's see how it reacts during the next rains, if any problems then I'll replace it.

I put a liberal helping of butyl sikaflex-stuff along the windscreen to maybe help prevent further ingress.