i just pulled mine last summer, and nearly all 95 mustangs had a small amplifier. Crutchfield and any other reputable audio supplier SHOULD have a bypass harness that lets you pull the speaker-out line from the back of the amp and bypass it. Receivers from back then didnt have quite enough juice to power speakers, so a 10-watt amp or so was installed to cut down on heat. do NOT run through the plugs directly on the old radio, you WILL blow your speakers, as the previous owner did to mine. Actually ended up replacing all four of them and running my own wiring because he effed it all up so bad.
as a side note... The air bag diagnostic module is right behind the amp. be careful when you work back there, even if the battery is unplugged, and make sure you take out the right part haha