I'd remove and reinstall the fuses/relays/circuit breakers under the hood. It sounds like one supply is corroded. The issues you are seeing don't sound pcm related.

Horn doesn't run through the pcm at all
Security light will, but you could have a burnt bulb
Door lock/unlock, is programmable,...read owners manual and reprogram. It's in high security mode. (for chicks)
Third brake light, oh isn't that the cruise fuse? I forget...Check fuses and bulbs.