door unlock, 92 chevy pickup

Is that the Designtech that doesn't have any doorlock outputs? Yes, you're right, the factory-alarm-disarm output could be used to trigger a relay to unlock the doors. The horn output is very brief, and probably would not stay on long enough to unlock. That truck has a 5-wire doorlock system; you'll need a relay, and you'll have to look at the various diagrams on this website to learn how to do a 5-wire (also called "reversing polarity") unlock. The easiest place to get to the doorlock wires in that truck, is by removing the left front speaker. There is a thick harness just under the speaker, and the lock wires are in there.