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. http://www.google.com/imgres?imgurl=http://www.justanswer.com/uploads/carhelp4u/2007-08-25_193738_northstar_spark_plug.gif&imgrefurl=http://www.justanswer.com/car/0tvh7-change-spark-plugs-2003-cadillac-deville.html&usg=__sDCm8VjdSGfmvCiUiCXW8cht5lQ=&h=637&w=678&sz=9&hl=en&start=9&zoom=1&tbnid=FkYQ58XFl7JV1M:&tbnh=131&tbnw=139&ei=tFcbTobTCKLCsQLppb3BBw&prev=/search%3Fq%3D2003%2Bcadillac%2Bdeville%2Bspark%2Bplug%2Blocation%26hl%3Den%26sa%3DX%26rlz%3D1C1ASAA_enUS427%26biw%3D1280%26bih%3D685%26tbm%3Disch&itbs=1 it is easy to do the front but a tight squeeze to get to the back. Good luck.