One problem related to brake light has been reported for the 1998 Cadillac STS. The most recently reported issues are listed below.
After a police stop for high-mount brake light inoperative, I learned that there is a 100% failure rate for this 3rd brake light in Cadillac vehicles. Mine is a 1998 Sts, now with 80k miles. It is not simply a bulb replacement because it is a sealed led unit and it is very expensive. The unit is subject to water damage. The price for the part alone is over $800. Research has revealed that this problem was first noted on 1992 - 1997 vehicles, and then again on 1998 - 2001 Cadillac vehicles with 100% failure rate, no repair is possible other than replacing the $800 part. The Cadillac division of gm should stand behind their product and replace these defective units.