How to fix keyless entry after replacing stock radio with aftermarket radio?

My girlfriend has a 97 honda civic ex. She bought an aftermarket radio and had me install it. When I was finished installing the radio, we noticed that the keyless fab was not working. (It was working before I started replacing the radio). The doors will lock via the door switch. My question is how do I get the keyless fab to work again. And the battery for the fab is not the problem.