Central locking: Random double horn and interior lights when driving
 

Driving home today there were a few times where the horn would double beep,and the dome light would come on. I assume it has something to do with the central locking. Is there a guide to trouble shooting this?

Not sure if it is related but I've been having intermittent problems with the passenger door sensor (the central locking) where the windows dont always roll down when the door is open. I rarely have passengers so I haven't gotten around to fixing that yet.

And normally when I unlock the doors the instrument panel will light up, (displaying the odometer) but today it didn't turn on until i put in the key.

HAve you gotten water in the car? The alarm control is under the drivers seat and if it gets wet, well you will be in trouble. Check that and the plugs that go to the alarm box. Unless you have tiny and paperclip sized arms, you have to lift the seat :-(

It is a grounding problem.
Search for the issue jcs identified.
Drill a drain hole at the lowest point.
Clear roof drains
Fit a ziploc bggie around the alarm after it has been dried out and is confirmed 'good'.

No water around the car recently. Last wash was 3+ weeks ago, and weather has been completely dry since then.