Electrical problems of this sort can originate from many different areas. The first question I always ask before diagnosing this type of problem is, ôHas anyone installed an aftermarket accessory?ö
If so, thatÆs were I head first. As an example, is that security system factory or aftermarket?
Test the alternator and battery as already suggested. A malfunctioning diode can create strange electrical problems. My guess is that you have a loose or corroded ground connection.
Look at a wiring schematic and check all grounds that affect the circuits involved. This includes direct connections and harness connectors.
Conner