Turn the ignition ON and energize the heater switch. Listen down under and in front of the seat pad: is there a click every 4 - 5 seconds? If not, maybe a fuse or connector. If so, one of the 3 or 4 heater elements is burned out, (the click is a circuit breaker resetting) which calls for entire seat removal from the car and disassembly of the leather surface and foam padding to get to the burned element section. You'll see it because of a burn ring at the break. Some have had good luck with jumping a short piece of wire across the break with solder, and others replace the entire heater with a GM replacement kit.
A GM dealer will get above $300 for the job. You can do it for the cost of a proper kit - there are several different heater arrangements, so you must know which seats you have - and you can do it with tools and a lot of work.
If you go back six months or so in here, and down in Electrical, there are other threads on seat heaters with photos of heater element break repair as well as the different types of heater pads. Try using the search function in the blue bar ^^^ or type "cadillac forums seville seat heater" in Google - or "cadillac forums seville ....... ........." similar words of interest
All the heater arrangements are in a GM/Helm Shop Manual (
www.helminc.com or eBay or subscribe your car to
www.alldatadiy.com)
'99 STS seat heater not working 
