Is the fuse for alternator a 25 amp on 98 F-150?

There's no single "alternator fuse" in a 1998 Ford F-150. The alternator's output is regulated by the alternator itself and isn't protected by a single fuse in the same way a circuit for accessories might be. Problems with the alternator typically involve the alternator itself, its wiring, or the battery charging system.

While there might be fuses related to circuits *powered* by the alternator (like the radio or interior lights), a blown fuse in those circuits would not directly indicate an alternator problem, it would indicate a problem with that specific circuit.

If you suspect an alternator issue, you'll need to have the charging system tested by a mechanic. They'll use a multimeter to check the alternator's output voltage.