To find the correct fuse, you must consult your owner's manual. The manual will have a fuse box diagram clearly showing which fuse is for the fuel pump.
If you don't have the owner's manual, you can try finding a digital version online through a site like Ford's website (as Mercury was a Ford brand) or a third-party manual provider. Searching for "1989 Mercury Grand Marquis owner's manual" should yield results.
Alternatively, the fuse box itself often has a diagram printed on its cover or inside the lid. This diagram may provide the information you need. However, the diagram's clarity can vary.