They should be under the engine cover. I found this which should help you. http://wiki.answers.com/Q/How_to_replace_spark_plugs_on_2003_cadillac_deville It may not look familiar because they used a coilover coil pack on your car. Basically instead of a central coil pack with plug wires running to the plugs, there is a plastic thing with a short boot on the bottom. you probably wont even see the boot until you remove the coilover. This pic should give you a better idea of the assembly.
it is easy to do the front but a tight squeeze to get to the back. Good luck.