2004 Elantra GT. Factory radio died, and I wish to replace with an aftermarket unit. Seems like the security system is built into the stock radio. Any issues with replacement ? Not bothered if security system not working after change. Thanks

 

When the security system is not working correctly, either due to incorrect function or information rec'd, the vehicle usually will not be allowed to start.

Ya just can't get away with it that easy. lol
Radios were not usually part of a CAN (Controller Area Network) system until 2008, but some manufacturers used other means.
I would check with a dealership Service Dept to be sure.