I had the same problem with my 99 STS. Do you have a 10 inch subwoofer in your back dash? If so you can use the speakers wires from it for a singal to your amp. If your amp does not take a direct speaker cable like that you can buy a converter that will convert the speaker wires to rca so you can hook your amp up that way. If your seville didnt come with a sub in the back dash you can run wire from your back door speakers.