My car will not stay charged. does the 1998 mecury grand marquis 4.6 have a 175 amp mega fuse. is so, where is it located.

Some possibilities, battery--battery connections--generator--generator wiring-- too much parasitic drain with everything off.

Here is charging system diagram. I didn't look for mega fuse, I will if you want it. In the diagram, you can see applicable fuses and fusible links.
If battery is down get it charged up, can't do any testing if battery is down. With a charged battery, everything thing off, use digital multimeter across battery posts, the generic spec for a full charge is around 12.6 volts. Then start vehicle, take second reading across battery posts, the second reading should be higher than the first if charging system is working.

Hi Kristin, I am not sure of the fuse location, but first have you checked the battery? how old is it, are the terminal posts and cables clean and tight. Is the alternator charging properly?. it could be you have a light staying on and draining the battery over time! the likely culprits in that case are the glove compartment and/or the light in the trunk.