The following suggetions are quoted from:
"How to Understand, Service and Modify Ford Fuel Injection & Electonic Engine Control" by: Charles O. Probst, SAE
Page 232:

When testing components, owners and shops without the necessary BreakOut Box (BOB), may choose to backprobe. Backprobers push a straight pin in through the back of a connector, probing for a contact that allows making an electrical test (voltage/ground/resistance) on that circuit.

CAUTION -- Be very careful when backprobing conectors. Backprobing can destroy the waterproofing of a connector. Moisture entering the opening can corrode the contacts inside the connector. If in doubt, don't backprobe.

When backprobing connectors, insert pins from the harness side.

Never force the probe into a female connector. On the male pin
probe the pin. Be sure you do not short between the pins.

Connect or disconnect multiple connectors and test leads only with the
the ignition off. Switch multimeter functions or measurement ranges
only with the test leads disconnected.

Do not use a test lamp that has a normal incandescent bulb to test
circuits containing electronic components. Use only an LED (light
emitting diode) test lamp.

Do not use an analog (swing-needle) meter to check circuit resistance or
continuity on electronic (solid state) components. Use only a high
quality digital multimeter having high input impedance (at least 10
megohm). jd

 

I have the same book and found it to be a great tool for understanding and repairing my truck.