I used to install alarms. GM are a pain compared to most vehicles in that you can't just send a signal to the switch wires. You'll need 2 Bosch style relays.
IIRC the wiring was
85 to remote controller, 1 to lock and 1 to unlock
86 to power (assuming the remote sends a ground signal)
30 to actuator positive, relay 2 30 to actuator negative
87a to ground
87 to power
You need to send the actuator a positive and a negative.
I just found a picture that also describes it - almost the same as what I said (85 & 86 are interchangeable)
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Correction: you got me thinking about this, it's been like 30 years, I think you needed to interrupt the wire from the switch to the solenoid. Still two relays but I think 87a goes to the switch side and 30 goes to the solenoid side. You can tell if the solenoid lines are grounded by default. If you cut one of the lines and apply power only to the solenoid side it locks. I highly recommend you roll the windows down before doing this. I can't tell you how many installers locked the keys in the car while adding door locks.