When winter comes we start to notice things like this
If you are using sealed beam lamps, bear in mind that these are tungsten. Then consider the difference between a 60 Watt tungsten light bulb in your house, and a 40-Watt flourescent tube. You could consider converting to bulb headlamps using the modern white-light 55/60 bulbs. I can personally recommend this. The downside is that a single stone chip or crack will require the reflector unit replacing for your next MoT, and they cost twice as much as sealed-beam units.
Regardless of the type of headlamp, have you had the alignment checked ? The MoT test only checks that your lights will not dazzle other road users, not that they are set for optimum night-driving. There has been a long-term fashion for lowering the front of the Scimitar, but not everybody remembers to re-align the headlamps after doing this. Go and have them checked by a garage.
Are you still running the old Coupe/SE5/SE5A no relay system ? If so, the voltage drop and current loss through the yards of wiring from the battery to the switch to the column control back to the fuse box and then to the lights can significantly reduce the current available at the bulb itself. Consider converting to a pair of relays close to the fuse box, and also check the earth connections from each headlamp (you can put as many volts on to the headlamp bulb as you like, but they've all got to get out of the other side back to the chassis.)
In damp or wet weather, the heat from the headlamps will bake the drizzle and road muck onto the headlamps with the effect that after 20 minutes driving the emmitted light can be cut by up to (a subjective) 50%. Stop in a layby and clean the lights with a wet cloth. As soon as I can develop a reliable and effective headlamp wash system I will be marketing it.
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Assuming that you've not blown the fuse, the heated rear window can suffer from earth problems, just like the lights. Having volts at one side of the heating element with respect to the other is one thing, but if the earth connection is poor not enough current can flow through the strips to make any temperature change. Check all the earth wire joints all the way down to the connection to the chassis.
If you've just lost some of the heating strips, Tandy and similar electronic shops sell a silver conductive paint for repairing circuit board tracks. Painting a small amount of this on to the defective track might help.
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You will need a test instrument in order to track down a current drain. I use an old Maestro boot light with two long leads, each with a crocodile clip attached, and an indicator bulb. If the current drain is several amps, this device not only shows it by the bulb illuminating, but it prevents you from burning out a very expensive test meter which normally only likes a few milliamps flowing through it. The bulb also acts as a current limiter, a 24W indicator bulb will allow a maximum of 2 amps to flow.
First, test that the bulb will light when the crocodile leads are placed on the battery terminals.
Turn the ignition off, and close the doors and rear hatch. Disconnect the positive lead from the battery. Put one crocodile clip onto the battery positive, and the other onto the positive lead. The bulb will light if the current drain is greater than half an amp.
Pull the fuses one at a time from the fusebox, replacing each one that didn't cause the lamp to go out. If none of the fuses caused the lamp to go out, move your attention to the few items in the car which are not fitted to a fused circuit. For example, test the alternator diodes by disconnecting the alternator plug, reconnecting the battery positive lead, and seeing if the test lamp lights between any of the alternator terminals and the plug.
If the bulb didn't light at all, but did light when each lead was connected to the battery in the initial test, use a multimeter in place of the test lamp with the multiswitch set to the highest current scale available, then step down until you get a decent needle deflection. Repeat the exercise with the fuses until the drain is found.
Having identified a particular fuse, (or clump of unfused wires), identify from the wiring diagram what current paths fan out from that fuse, and disconnect the current sources one at a time until the culprit is isolated. For example, on an SE5A, where the top fuse feeds the courtesy light switches and interior lights, bonnet light, cigar lighter socket, clock, headlamp flash, and horn, disconnect each of these items in turn to see if you can isolate the culprit. If this fails, slacken the terminal screw on the fusebox outlet side and take each of the three wires out in turn until you can isolate the current path. Then, with the guilty wire removed, see what has stopped working, and try tracing along the wire to various junction points.
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