Ford 1999 Explorer Owner's Manual page 194

Roadside emergencies
3. Connect the negative (-) cable to the negative (-) terminal of the
assisting battery.
4. Make the final connection of the negative (-) cable to an exposed
metal part of the stalled vehicle's engine, away from the battery and the
carburetor.
The preferred locations of an exposed metal part (to ground the circuit)
are the alternator mounting brackets or an engine lifting eye. Do not
use fuel lines, engine rocker covers or the intake manifold as grounding
points.
Do not connect the end of the second cable to the negative (-)
terminal of the battery to be jumped. A spark may cause an
explosion of the gases that surround the battery.
• Engines except 4.0L SOHC
194