I'm not in a cold climate. I don't have to worry about anything other than poor quality fuel. I keep a spare filter and PS 911 under the back seat just in case.
That said, have you tested the fuel heater to verify that it's actually working? Here is an excert from my '01 service manual:
(I tried to copy the pictures, but was not successful)
FUEL HEATER
DESCRIPTION
The fuel heater assembly is located on the side of
the fuel filter housing (Fig. 9).
The heater/element assembly is equipped with a
temperature sensor (thermostat) that senses fuel
temperature. This sensor is attached to the fuel heater/
element assembly.
OPERATION
The fuel heater is used to prevent diesel fuel from
waxing during cold weather operation.
When the temperature is below 45 68 degrees F,
the temperature sensor allows current to flow to the
heater element warming the fuel. When the temperature
is above 75 68 degrees F, the sensor stops current
flow to the heater element.
Battery voltage to operate the fuel heater element
is supplied from the ignition switch and through the
fuel heater relay. Also refer to Fuel Heater Relay.
The fuel heater element and fuel heater relay
are not computer controlled.
The heater element operates on 12 volts, 300 watts
at 0 degrees F.
DIAGNOSIS AND TESTING - FUEL HEATER
The fuel heater is used to prevent diesel fuel from
waxing during cold weather operation.
NOTE: The fuel heater element, fuel heater relay
and fuel heater temperature sensor are not controlled
by the Powertrain Control Module (PCM).
A malfunctioning fuel heater can cause a wax
build-up in the fuel filter/water separator. Wax
build-up in the filter/separator can cause engine
starting problems and prevent the engine from revving
up. It can also cause blue or white fog-like
exhaust. If the heater is not operating in cold temperatures,
the engine may not operate due to fuel
waxing.
The fuel heater assembly is located on the side of
the fuel filter housing (Fig. 10).
The heater assembly is equipped with a built-in
fuel temperature sensor (thermostat) that senses fuel
temperature. When fuel temperature drops below 45
degrees 6 8 degrees F, the sensor allows current to
flow to the built-in heater element to warm the fuel.
When fuel temperature rises above 75 degrees 6 8
degrees F, the sensor stops current flow to the heater
element (circuit is open).
Voltage to operate the fuel heater element is supplied
from the ignition switch, through the fuel
heater relay (also refer to Fuel Heater Relay), to the
fuel temperature sensor and on to the fuel heater element.
The heater element operates on 12 volts, 300 watts
at 0 degrees F. As temperature increases, power
requirements decrease.
A minimum of 7 volts is required to operate the
fuel heater. The resistance value of the heater element
is less than 1 ohm (cold) and up to 1000 ohms
warm.
TESTING
(1) Disconnect electrical connector at sensor (Fig.
10).
Turn key to ON position. 12 volts should be
present at red wire. If not, check fuel heater relay
and related wiring. Refer to Relay Test—Fuel Heater.
If OK, proceed.
Turn key OFF. Check black wire in connector for
ground continuity with an ohmmeter. If continuity is
not present, correct open ground circuit. This test can
also be performed with a voltmeter by backprobing
black wire with it connected to sensor. Reconnect
electrical connector and turn key ON. Voltage drop
should not exceed 2 volts (2 volts lower than checked
at 12V+ connector). If voltage is lower, check for dirty
or corroded ground connection and repair. If OK, proceed.
(2) With electrical connector disconnected at sensor
and key OFF, check electrical/mechanical operation
of fuel temperature sensor. Proceed to next step:
(3) Using an ohmmeter, check for continuity across
two terminals in electrical connector at side of sensor.
Sensor circuit should be open if fuel temperature
has risen above 75 degrees 6 8 degrees F. Sensor circuit
should be closed if fuel temperature has dropped
below 45 degrees 6 8 degrees F. If not, replace fuel
heater assembly. This same test can also be performed
using a voltmeter, with key ON, and by backprobing
connector.