Well, finally I got a Disco V8 -99 with BCU issues, I think...
Turn signal, front sunroof, c-locks, most of the powerwindows, heated screens, rear wiper etc doesn't work. I spent an hour going through the car with my Hawkeye with no luck. No faultcodes and most of the functions doesn't pop on even if I try to force them with the hawkeye.
I looked ate the fuses and they're fine. Groundpoints seames okey.

Now, I have an Disco TD5 -99 in parts, can I use that BCU and how do I install it?

 

I believe the IDM on the fuse box will re-learn the new BCU, but the ECU and BCU may have to be introduced by the Hawkeye or another device. Other than that it is like any other computer, plug and pray. May help if you have key fob available. Can't comment on a BCU from a TD5, not too many of them in US. Maybe program options in the BCM to make it work for both (like fuel shutoff solenoid, etc.).