Did you check on a factory wiring diagram and remove the rear seat in order to check that it is the security relay and not some other relay?
Did you pull the dash panels to see how the aftermarket radio is wired?
Your question is a good one but there are various ways that an aftermarket radio could be wired, and many more relays under the rear seat than one.
So you might make more prograssfaster by getting a FSM - EBay sellers often have them, they get them from recycling yards, dealers that are getting rid of them (GM has put the info on line for dealers) or individuals.
Albertj