Keyless entry doesnt work anymore...

Well aint this a B! I got this used Honda Civic (1997 EX 4dr), and it had keyless entry that worked. Anyway, I remember reading about how if you were to take out the Honda brand radio the keyless entry would not work anymore. BUT, like I said when I bought the car, it ALREADY had an aftermarket radio in it (sony) AND the keyless entry STILL worked. Moving on, I figured "well, since the keyless entry still works with this current Sony radio in it, I will replace it (since it was 3 years old) and put in a newer one (see pic below) and keyless entry will still work." Well, I was wrong! Right after I had the kind people at Best Buy install my new radio/cd player, keyless entry doesnt work anymore! Why is this? Why would it work before with the other Sony radio, but not with this new one I put in? I actually went down to Santa Monica Honda and the mechanic said that it has to do with something reseting...? I wasnt paying much attention. If anyone know why it doesnt work anymore, please let me know. Can it be made to work again?