OK not posted on this thread yet as nothing useful to contribute till this morning, bear in mind this a Tezza not an IS but similar symptoms. The car also has has an after market TOAD alarm/immobiliser.
List of things U/S
Fuel Gauge (Flashing Light)
Temp Gauge
Rear Defogger
Outside air temp indicator
Air Con on Automatic (Worked on manual)
Tiptronic Gear indication
Snow button
ECT Power button
Charging indicator light on dash
Door Open Indication on dash (Added)
+ more I did not even notice probably
Cutting a long story short I started the car and hit the horn by accident putting my seat belt on this morning and the effin horn stayed on, popped the bonnet and pulled the horn fuse and the horn kept going. Next I took out the horn relay and finally the horn stopped Thank FFT I thought. Then what now, FFS the engine had stopped so put the fuse back in, lid back on the fuse box and shut the bonnet. Went to open the drivers door and it's locked, keys in the ignition and turned on, with the demister fan running (as it was on manual to defog the screen)! The car had locked all the doors!
Got the spare key after a while (another long story) and got the car unlocked, fired it up and well B***ER me all the stuff on the list above was working as normal. The only difference is that the horn relay is still out.
Ploughed through a the wiring diagrams and find that the horn fuse also provides a feed to the "theft deterrent ECU", the horn wiring is a simple ground through a switch. Tested the relay and it's OK so the only way to ground is through the "theft deterrent ECU"!
As pointed out above the "theft deterrent ECU" also controls the headlights, it's also connected to the "body ECU" (2 of) that control the rear demister plus other stuff.
That's far as i've got so far but will post back when I have any more.