What can cause all the brake lights not to work on a 1998 mountaineer after you have checked the fuses and bulbs?

Two things to check first: The brake light switch may not be functioning properly. Check connections and make sure it's adjusted if applicable. If you have a meter, check for continuity when you step on the pedal. Second, if it's not the brake light it could also be the turn signal switch. Yes, that's right. When a turn signal switch fails it can also take out the brake lights, but you'll need to thoroughly verify that before you tear into your steering column, and if you have air bags, DON'T tear into your steering column.