Is the fuel pump in a 1998 oldsmobile silhouette in a trap door in back where spare tire would be

SOURCE: ECM location 1998 Oldsmobile Silhouette

On the 98 Silhouette, the PCM should be located in the air filter housing.

SOURCE: Gas Gauge not working properly

First...how do you get 25 mpg...I get 21 or so!!!!  Additives will never work on this.  The price for the pump is $150-175 for an aftermarket unit.
I just did the tank job for the sender.  Get an aftermarket pump/sender.  DO NOT ORDER from RockAuto for this part...their listing is wrong.  They send the gasket...which comes with the replacement.  Replacing the unit is relatively easy:

• Have less than 5 gals of gas in the tank.
• Get the driver's side of the car in the air and support it.  
• Remove plugs to the left of the tank on the bottom of the vehicle
• Start car and allow gas pressure to subside...car will stall
• Remove the fill pipe from the left rear of the tank..there is a valve on the fill pipe which will contain all but a few drops of gas
• support the tank and remove the bolts that hold the tank straps and lower the tank.
• lower the tank and remove the vent hose at the rear of the tank so you can swing it out
• after swinging it out, prepare to remove the unit from the top of the tank retaining the connector snap ons for the new unit.  Clean the area around the pump to remove the possibility of contamination.  Tap the ring sharply counterclockwise to remove it from the tank.
• Remove old and install new, pressing the unit into the tank...be sure to use that new gaaket they gave you.  Tap on ring, swing tank back, reconnect hoses, raise and bolt up.  Plug in.
• Start the car after turning key and waiting for 15 seconds to start so pressure can build.

SOURCE: 1999 oldsmobile silhouette van won't start. no power to fuel pump

I have a silhouette and it had the same problem
Follow the fuel pump wiring harness to where it plugs into the floor of the van--pull the connectors apart--they will probably be corroded, and you will possibly have to change both wiring harnesses or connectors.

SOURCE: Replacing mirror on 1998 Oldsmobile Silhouette

you'll need to remove your door panel (screws are by the door handle, arm rest, under door and side of door) and remove the cover adjacent to your mirror to remove the bolts to remove the mirror.

SOURCE: how to change rear spark plugs 1998 oldsmobile

First you'll have to rotate the engine forward to get at the rear plugs, to do this remove the the air cleaner, intake air plumbing, and front engine stabilizers. Disconnect the exhaust flange from the engine to the exhaust pipe, from underneath the car remember to use a good set of jack stands on stable ground, when you're working under a ton or two of sharp metal you can never be too safe. Remove the upper radiator hose then put a come-a-long or ratchet tie down ******** the engine lift point and to one of the bolts left after removing the stabalizers, then winch the engine forward exposing more of the rear of the engine. As I remember you'll have to remove the EGR Valve, possibly the upper intake manifold, the ignition coil pack and the canister vent valve which mounts on the back of the coils, getting at these rear bolts is a bear, I wouldn't replace them just put the vent valve on the studs the ignition coil on top of that and let the front bolts hold the ignition coils down, it's worked fine for me for over 100 K Mi. Heed the next step, it's important so you don't have to go through the rest of your life known as Lefty. Before you put your arms behind the engine, double up the tie down strap, if one falls off it'll break and pin your arms behind the engine, safety again. As you can see this is a relatively involved process, more from the number of steps than any complexity. There are many hidden bolts and number or mark all cables you remove although none will fit any of the plugs except where their suppose to go. Take pictures, and do everything that needs to be done in the back, don't be like me and not change the O2 Sensor trying to save $50 just to have to do it all over again in 6 months. Good luck.