I Have a 1998 honda civic and i want to install a key less entry. But i have an aftermarket radio can it work ?

The 1998 may or may not have come with keyless entry (see the first source for how to determine what yours is) and I think 1999 was the first year it was put in the radio, and that it was only in the EX trim the first few years. If you call that vendor they can probably clue you in whether the radio was involved in your keyless entry (if you have one). They have been helpful to me. Anyway, you can certainly add an aftermarket system. The second source has the car-side wiring info. Notice that it calls for a couple connections to the anti-theft module behind the radio, an indication that the radio has nothing to do with it in yours.