Electric seats
and electric windows - way out!
Whenever I've had electrical problems, particularly involving strange instrument readings, it's been a bad earth. Without seeing a wiring diagram, it's impossible to tell, but I could imagine that the electric windows link into the alarm system, which I know links to the speedo, as it has the red "armed" light in it. The alarm box is behind the instruments and takes all its power and wiring from the loom that goes back, through the bulkhead and underneath to the ECU under the driver's seat. Corrosion under there is almost a design feature, so it might be worth checking the condition of the connectors - there's the astonishing "oil in the loom" problem under there, too, so it might be worth checking how that's going. Try running a new wire directly from the battery earth to one of the ECU mounting screws and see if that improves things.
Thinking about it, I had an issue where bad after-market wiring in my headlight circuit created a voltage drop that threw the alarm and tripped the immobiliser - the solution there was to run a new power-feed to the headlight switch, but that wasn't ideal... If there's a bad connection in your heated seat circuit that somehow the alarm's picking up on as a voltage-drop, that might cause it to do weird things like stop the speedo working properly. Seeing as the seats are after-market, I'd definitely check how they're plumbed in and that proper relays have been used to power them, etc. Try disconnecting them altogether and see if the problem recurs - fault-finding's all about systematically isolating the cause...
Welcome to the wonderful world of Defender electrics - I had a problem once where I had to keep pressure on the handbrake when the engine was at idle, or it would stall - turned out the ECU was earthing through the handbrake cable, despite the earth-straps from the seatbox to the chassis looking fine...
Good luck with it - let us know what you find
