Electrics: Volvo XC90, 2007 Horn on, limp mode, many faults

[dwair]

Volvo XC90 D5 - 2007 Horn continually on, limp mode, many faults...

Horn is on continually if the fuse is in place. I have had a look at the CEM in place and there doesn't seem to be any obvious signs of water damage. Looking at the wiring diagram there seems to be a relay/switch in the CEM that I guess is no longer staying open.

Water may well have got into the yawl sensor under the seat by the amp about 6-8 months ago. There are signs of corrosion on the amp and some of the wiring in that area.

I put it on an icarsoft scanner and came up with an alarming number of errors:
500F State: None Air Condition, Faulty signal
7700 State: None Engine control module (ECM)
18B0 State: None Swirl actuator control, Offset fault
1517 State: None Turbo control, Pressure too high
9140 State: None Throttle module, Faulty signal
9190 State: None Throttle module, Faulty signal
0A31 State: None Control module communication, Signal missing from Engine Control Module (ECM)
0A31 State: None Control module communication, Signal missing from Engine Control Module (ECM)
0A31 State: None Control module communication, Signal missing from Engine Control Module (ECM)
0040 State: None Air quality sensor, Internal fault
0004 State: None Siren, Communication fault
000E State: None Steering wheel angle sensor, Internal fault (this was a bit iffy before)
1F00 State: None ECM, software, Faulty signal

Where do I start?

Any help appreciated!