The antenna is behind the dash, above the glove box. You can see it if you remove the side panel(s) next to the radio head unit. Where it goes from there I'm not sure, but the receiver itself may be part of the ETACS or BCM. Why do you think it needs replacing?

 

No remote key came with the Kia. Only 1 regular key. I was told that it had a keyless entry system after checking the vin #.
I was told it would be cheaper to install new one.

 

I was able to get replacement keys from https://www.keylessremotewarehouse.com/ and they saved my butt!