Because it's not always in the same precise spot, your best bet is to:
1. Consult your owner's manual: This is the most definitive resource.
2. Look under the dash thoroughly: Use a flashlight and carefully examine the area near the steering column and fuse box. The switch itself is usually a small, red button or square-shaped device. Some are labeled, others are not.
3. Search online: Specific forum posts for 1987 Ford Broncos might contain images or descriptions of the location for that year and model. Be sure to specify the 302 engine and automatic transmission as this could influence its location slightly.
If you still can't find it, consider seeking assistance from a mechanic.