For all the people who have a Proton Persona and have problems with their immobiliser or their horn, here are my experiences of both.
Symptom
Car will not start. Lights on dash come on momentarily then go off.
Resolution
The immobiliser was operating and preventing the car from starting. The problem had been intermittent but one day the car refused to start. The immobiliser is under a plastic cover to the right of the driver's footwell. A coil goes from the immobiliser up round the ignition barrel. The coil was not fastened properly and has slipped further down the ignition barrel. I moved the coil back to the top of the ignition and the fault went away.
Symptom
Horn will not work.
Resolution
Fixing the horn took some time. The horns (two off) are located behind the front bumper, just under the headlights and are really hard to get to.
The horns are always live with the steering wheel switch making the connection to earth. First check the fuse in the drivers footwell, it should be a 10A fuse. If the fuse is ok then check the cabling to the steering wheel. Remove the lower plastic from under the steering wheel, it is fixed on with two posidrive screws accessed by lying in the footwell.
Once the cover is removed, you should see two strange cables leading upto the steering wheel. One has two cables and an orange connector, this is the airbag cable. The other cable has a grey connector and a green wire coming out of the wiring loom. The green wire is the live. To test the horns, disconnect the connector and push a thin piece of wire into the connector (there is only one copper connector in the socket). Short the other end of your piece of wire on the metal surround of the ignition. The wire will complete the circuit and the horn should sound. If it doesn't then the problem is in the wiring loom, remove the fuse for the horn again and using a multimeter measure the voltage between the lower fuse connector and earth. If there is still no voltage, it has to be the wire between the fuse box and the battery.
Ok so the horn sounded when you shorted the wire between the grey connector and the metal surround of the ignition. So the problem is most likely in the cable upto the steering wheel or in the brushes connecting the steering wheel to the steering column.
I would advise getting a mechanic to check out the steering wheel connections as the airbag has to be disturbed and it could cause a safety issue.