How to REconnect my keyless entry? Chevy Trailblazer?

Ok, i bought a 2003 chevy trailblazer and the guy that sold it to me didn't give me the keyless entry remote, but i noticed the car has an after market alarm, (which I also didn't receive the remote to) so i bought the STOCK keyless entry remote. I followed all the steps to program it but it doesn't work. I'm guessing they disconnected the stock keyless entry in order to make it work with the aftermarket alarm, now i wanna know how to connect it back. If anyone can help I would really appreciate it!!!