I know on my explorer, I can not arm/disarm the factory security system when I have my camaro running within 20 feet of the explorer, even if I'm standing right next to the truck. The Sick ignition system I'm running in the camaro ( MSD based ) interferes with the remotes signal, ( funny thing is, I have NO ignition noise in the Camaro's stereo system at all ) but, I can arm/disarm the Ex's factory system usually with no problem from about 80 yards away from the truck on any given day, as long as the camaro is OFF!
So RF interference is quite possible.
P.S. In my Ex, if the alarm goes off, all I have to do is turn the ignition on and it disarms, allowing the truck to start, and I DON'T have the "PATS" system being it is a '95, go figure, thought the truck isn't suposed to start at all unless alarm is disarmed. A slaphammer and slimjim are all one would need to steal a suposedly "secure" truck, so much for that wonderful "starter interrupt circuit"...
Seeing that if the ignition is turned "ON" while the alarm is going off, it DISARMS the system, and a ford ignition can be slapped out in less than 5 seconds if you know what your doing...
What WAS Ford thinking? Installing a starter interrupt that gets disabled when the ignition is turned on...another one of their "better ideas" heh heh