What other makes and models are interchangeable with the fuel sending unit for 1999 Pontiac Bonneville?

In the fuel line back by the gas tank on the passengers side of the car just in front of the rear wheel.

Check fuses then check this link:
https://www.google.com/search?q=1999+pontiac+bonneville+fuel+pump+fuse&oq=1999+pontiac+bonneville+fuel+pump+fuse&aqs=chrome..69i57.18806j0j8&sourceid=chrome&es_sm=122&ie=UTF-8

You have no fuel safety shut off switch
They are only on ford products

Are you referring to the fuel tank level sensor for the
dash fuel gauge?

If you go to Rock Auto.Com-- you will see that the sending
unit is about $50.00

The mechanic is really screwing you over for sure

Drop the fuel tank & replace yourself

Did this someone check it or just tell you it needed a sensor?
On some vehicles there is' a sensor on the radiator but most are in the plastic recovery tank. Just fill the tank.The coolant needs to touch it for the light to go out.While the engine is cool add 50/50 mix to the coolant reservoir/recovery ,replace cover & start engine, Check light.
When fuel tank gauges start going wacky its because 1 of only a few things has happened.
The wiring to the fuel tank sending unit has become corroded or cut somehow, the plug has become loose or corroded.
The sending unit inside the tank was hit with a syphon hose or the tank took a good hit somehow. Maybe in an accident.
It is rare that the gauge has gone bad. Not to say it hasn't happened, it's just rare.

http://www.autozone.com/autozone/parts/Duralast-Sensor-Radiator-Coolant-Level/1999-Pontiac-Bonneville/_/N-jd6ghZ9pehj?counter=0&itemIdentifier=384818_0_0_

there is a relay for the ac and it is normally under the dash close to the ac evaporator

No it don't, to replace fuel pump, gas tank will have to come down.

Yes!! lots of patience, lines are not that easy especially routing them, check the sending unit for rusty lines and the tank also, nothing worse than reinstalling everything back only to find you have another leak

Inside the gas tank is the fuel sending unit. It is a floating device that is held on rails and it slides up and down depending on gas level and then sends a signal to the gas gauge. Either the float is leaky or it is stuck. The down side is you have to pull out the tank, then use brass punch and knock the ring counter-clockwise until it becomes loose. Then pull out and change the sending unit. Do make sure negative battery cable is disconnected and wiring harness and fuel lines are disconnected before pulling the tank.

Is the engine from the bonneville a 3.8 too. If its is it will interchange, Just make sure you use everything off the 98, like all the sensors ect. The basic engines are the same its the add ons that will be different.

The oil pressure sending unit is located behind the oil filter, Threaded into an aluminunm housing. It has wiring that plugs into the sending unit. The oil filter has to be removed and you will need a deep well socket and a 6 inch extension. Change the oil and the sending unit together.