I'm 99% sure that the Regal came standard with remote keyless entry. All you have to do:
-buy a remote
1) Sit in the drivers seat
2) Remove key from ignition cylinder
3) Close all the doors
4) Press and hold the UNLOCK on the door lock switch
5) While holding the door lock switch in the UNLOCK position, insert
and remove the ignition key twice. DO NOT ROTATE THE IGNITION CYLINDER.
6) Insert the ignition key a third time, leaving the key in the
ignition cylinder. The ignition MUST remain in the LOCK position for
the rest of the procedure.
7) Release the door lock switch. You will hear 3 chimes. This chime
feedback indicates the transmitter programming is active.
8) Press and hold the LOCK and UNLOCK buttons on the remote for 12
seconds. You will hear two chimes. This chime feedback indicates the
transmitter has been accepted.
9) Repeat step 8 for each additional transmitter
10) Remove ignition key from cylinder to exit programming.
11) Verify functions of remotes.
Here's a link to a remote. $29. http://www.carandtruckremotes.com/r012.html
Your RPO sheet should be on the left side underneath the spare tire cover.