I have been in the automotive field since 1975 and have accumulated over 25 years of experience with General Motors products achieving the designation as a GM Master Technician. I am also currently certified and an ASE Master Technician, and hold an Advanced California Smog Check License. I have been the lead technician and team leader at a multi-line dealership. After leaving the dealership I spent a couple of years working in an independent shop and now find myself applying my experience for your benefit at RepairPal. My training includes numerous GM classes and college courses which resulted in an AA degree in Automotive Technology.
The air bag control module has detected a fault with the system. Fault code(s) should be stored in the module. The code(s) will need to be retrieved, diagnosed, and repaired to keep the air bag light off..
It is possible that some sort of debris has gotten the drivers seat belt buckle. Whatever the reason the buckle is no longer working I would suggest you have it replaced for safety sake. In most cases the buckle can be replaced without replacing the belt also. It's generally not a very costly repair.
I am assuming you have a 4.6 liter northstar engine in your 1999 Cadillac Eldorado. If that is the case the coolant sensor should be located in the rear cylinder head under the coolant crossover manifold, not a very easy place to access.