car alarm, 77 chevy truck

Check to make sure you have a good ground path for the alarm. Back out the pin switches and use a piece of sandpaper to remove the old paint that comes in contact with the pin switch. The theory behind this wire is as soon as it sees a ground it triggers the alarm. This issue you are seeing seems to be a ground issue. When all the doors and hood are shut and you set the alarm do you get one chirp or do you get multible chirps. If you get one chirp this is a definate ground issue. Also check the crimped connectors that you used to attach the wires to the pin switches. A bad crimp can cause a faulty connection. I have seen this way to much. If you hear multiple chirps this means that there is a zone open, meaning that the brown trigger wire is grounded. Let me know if this helps