Yo George,
The grids need to be tested.
This is by Ford:
- Supply open or shorted.
- Ground open.
- Circuits open or shorted.
- Window regulator control switch.
Section 01-11: Glass, Frames and Mechanisms
1996 F-150, F-250, F-350, F-Super Duty and Bronco Workshop Manual
PINPOINT TEST E: REAR WINDOW WILL NOT DEFROST
E1 CHECK REAR WINDOW DEFROST SUPPLY
- Remove heated back window switch and light as outlined.
- Key to RUN.
- With test lamp connected to a known good ground, back-probe Circuit 8 (O/Y).
Did test lamp illuminate?
Yes
No
GO to
E2.
GO to
E3.
E2 CHECK REAR WINDOW DEFROST GROUND
- Using ohmmeter connected to known good ground, connect other lead to Circuit 57 (BK).
- Measure resistance.
Is resistance 5 ohms or less?
Yes
No
GO to
E3.
SERVICE open in Circuit 57 (BK). RESTORE vehicle. RETEST system.
The defrost ground wire has a ring terminal screwed to the body in the driver side taillight well.
E3 CHECK FUSE 7 REAR WINDOW DEFROST SUPPLY CIRCUIT
- Remove Fuse 7 (15A) at IP fuse panel.
- Using ohmmeter, check resistance between fuse output and Circuit 8 (O/Y).
Is resistance 5 ohms or less?
Yes
No
GO to
E4.
SERVICE open in Circuit 8 (O/Y). RESTORE vehicle. RETEST system.
E4 CHECK REAR WINDOW DEFROST SUPPLY FOR SHORT
- Using ohmmeter connected to known good ground, connect other lead to Circuit 8 (O/Y). Read resistance.
Is resistance 5 ohms or less?
Yes
No
SERVICE short to ground Circuit 8 (O/Y). REPLACE Fuse 7 (15A). RESTORE vehicle. RETEST system.
REINSTALL Fuse 7 (15A). GO to
E5.
E5 CHECK GRID FEED TO DEFROST RELAY
- Check output side of rear defrost fuse in power distribution box with test lamp.
Did test lamp illuminate?
Yes
No
GO to
E7.
GO to
E6.
E6 CHECK GRID SUPPLY FOR SHORT TO GROUND
- Using ohmmeter connected to known good ground, connect other lead to output side of rear defrost Fuse (30A).
Is resistance 5 ohms or less?
Yes
No
SERVICE short to ground Circuit 185 (BK). REPLACE rear defrost Fuse (30A). RESTORE vehicle. RETEST system.
GO to
E7.
E7 CHECK GRID SUPPLY FOR OPEN CIRCUIT
- Using test lamp connected to known good ground, connect other end to Circuit 185 (BK) at rear defrost relay.
Did test lamp illuminate?
Yes
No
GO to
E8.
SERVICE open in Circuit 185 (BK). RESTORE vehicle. RETEST system.
E8 CHECK RELAY OUTPUT TO GRID
- With test lamp still connected to known good ground, connect other end to Circuit 186 (BR/LB).
- Turn key to ON.
Did test lamp illuminate?
Yes
No
GO to
E9.
REPLACE heated back window switch and light. RESTORE vehicle. RETEST system.
E9 CHECK REAR WINDOW DEFROST GRID FEED
- Disconnect heated back window switch and light.
- Disconnect feed wire at rear window.
- Connect one lead of ohmmeter to known good ground; connect other lead to Circuit 186 (BR/LB).
- Measure resistance.
Is resistance 5 ohms or less?
Yes
No
SERVICE Circuit 186 (BR/LB) for short to ground. RESTORE vehicle. RETEST system.
GO to
E10.
E10 CHECK REAR WINDOW DEFROST GRID FEED FOR OPEN
- Using ohmmeter, connect one lead to Circuit 186 (BR/LB) at heated back window switch and light. Connect other lead to window connector.
- Measure resistance.
Is resistance 5 ohms or less?
Yes
No
GO to
E11.
SERVICE open in Circuit 186 (BR/LB). RESTORE vehicle. RETEST system.
E11 CHECK GRID GROUND
- Using ohmmeter, connect one lead to ground connector at grid.
- Connect other lead to known good ground.
- Measure resistance.
Is resistance 5 ohms or less?
Yes
No
System OK. RESTORE vehicle. RETEST system.
SERVICE open in Circuit 57 (BK) at rear window grid. RESTORE vehicle. RETEST system.
The Defrost grid has 2 wires. Each wire has this style connector on the end of the cable.
One wire is Hot for the defrost grid. The other wire is a black ground that connects to the frame under the bumper.
you can see here where the defrost grid connections connect.
defroster battery power wire is BR/LBHere is the 96 wiring diagram;
note, in Engine Compartment Fuse Box, aka Power Distribution Box, check **** fuse #14
Heated Back Window Switch Connector, Bronco
Pin Number
Circuit
Circuit Function
1
186 (BR/LB)
Heated Back Window Switch to Heated Back Window Grid
2
185 (BK)
Battery Feed to Heated Back Window Switch and Grid
3
8 (O/Y)
Heated Back Window Timing Circuit Feed
4
57 (BK)
Ground Circuit
5
19 (LB/R)
Instrument Panel Lamps Feed
Here is the removed harness showing Defroster Battery voltage and ground wires; clipped to the window bracket.
