The rear window defroster on my '95 850 wagon recently stopped working, and
during a rare above-zero day this weekend, I took the time to take the trim
pieces off and isolate the problem. It seems that the ground-side connector
(which is on the upper/passenger-side portion of the heating grid) had
developed a poor connection, and cooked/melted the connector. I assume that
this would be a relatively common failure mode for this system. I just thought
I'd summarize the debugging procedure for this problem for anyone else suffering
from a lost defroster:

1) check fuses (there is a 30A and a 15A fuse associated with the mirror/window
defrosters, in the main fuse box under the hood)

2) Check for 12v on the defrosting grid when switched on. Put one meter lead
anywhere on the car frame, and the other lead on some portion of the grid (the
grid is uninsulated metal deposited on the glass -- be careful not to scratch
off the metal!) If you don't have any voltage, then the problem is upstream
of the window (either the switch itself, the relay under the driver's foot
bolster, or a connector in between)

3) Assuming you find 12v on the grid, the problem is likely the ground
connection to the grid. Unfortunately, you'll have to remove some trim pieces.
You can start with the driver-side D-pillar to get at the main connector for
the rear-window harness (which includes wiper, lights, and the defroster), as
it is pretty simple to remove. Remove the speaker (pry out from inboard side
where there are small slits to insert a screw-driver). Then remove upper trim
piece, starting at speaker and moving up to roof and forward to C-pillar. The
connector is above the speaker. The 12v wire is the large diameter Yellow/Red
wire, and the ground is the large diameter black wire. Check for zero ohms
to frame from ground wire.

4) Assuming you find zero ohms to frame on black wire, you will have to remove
the tail-gate trim. Start by removing the plastic surround of the inside door
latch (use a putty knife to pry along lower edge). Then, carefully pry up on
bottom of tailgate trim to release four plastic clips (you will likely break
one or two, so purchase spares). Then slide entire panel upwards (towards top
of window) to release upper clips and remove the carpeted panel. Now you can
unclip the upper trim piece (starting at bottom) to reveal the grid connectors.
Ground connector is on passenger side, and if visual inspection shows it to be
melted, cut it off and replace (I tinned the wire with solder and used a screw
terminal connector, but a simple crimp-on connector is likely adequate).

As I said, this is probably a common failure mode for the rear-defrost, as the
connectors are exposed to high humidity from the window, and will eventually
corrode and develop a non-zero ohm resistance, at which point they will start
to heat up from the voltage-drop, causing the plastic connector to out-gas,
adding more crud to the connection, making it higher resistance, etc.

Hope that helps someone with misty windows!

grant..