The following wire diagrams show you how to wire up your aftermarket alarm  and allow it to co-exist with with factory alarm.

Every time, you press the arm button on your aftermarket alarm, it will arm your factory alarm; when you push disarm button on your aftermarket alarm, it will also disarm both factory and aftermarket alarm.

(input require (-) lock and unlock pulses from your aftermarket alarm.)

Get some general purpose diodes at your local hobby store (these are use to isolate all the wires from interfering with each other)