The fuel pump on your Audi A6 Quattro is located inside the fuel tank. My service information states that the fuel pump can be replaced without removing the fuel thank. Please use the following link to the RepairPal estimator to find our the probable cost for your fuel pump replacement - http://repairpal.com/estimator?car_type_id=2653
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.
What light is on and what chime is sounding?