A wing mirror that won't clear on a cold morning is one of those problems that seems straightforward but can have several different causes. Before you do anything else, work through this list in order - each step takes a couple of minutes and the most common causes are at the top.
Cause 1 - The mirror switch isn't in the heating position
On a lot of cars - particularly VAG group models like Volkswagen, Audi, Seat and Skoda - the mirror heating doesn't come on automatically when you press the rear demist button. There's a separate position on the mirror adjustment knob that activates the heating, usually the far right setting on a five-position knob.
If you've never had to use it before and assumed the demist button covered everything, it's worth checking the mirror controls first. It's an embarrassingly common fix - and a free one.
On most other cars, the rear demist button does activate the mirrors. Check your owner's manual if you're not sure how your car works.
Cause 2 - A blown fuse
If the switch position is correct and neither mirror is heating, the fuse is the next thing to check. Both mirrors are usually on the same fuse, which is why a blown fuse tends to take both out at once. If only one mirror has stopped working while the other still heats normally, it's almost certainly not the fuse - skip to cause 3.
The fuse box is usually under the dashboard on the driver's side, or in the engine bay near the battery. The relevant fuse is often labelled "heated mirrors", "mirror heating" or grouped under "rear demist" - check the owner's manual for the exact location and number.
Pull the fuse out and hold it to the light. A broken wire inside means it's blown - replace it with the same amperage. If the new fuse blows immediately, there's a short somewhere in the wiring and that's a garage job.
Cause 3 - Dirty or corroded connector pins
The heating element in the glass is powered through two small connector pins on the backing plate, which plug into the motor housing. Over time, those pins can develop corrosion or a film of oxidation that prevents good electrical contact - even when the connector looks properly seated.
Pull the glass away from the housing slightly, disconnect both pins, and look at the contact surfaces. Any greenish tinge or dull coating is corrosion. A light rub with fine sandpaper on both the pins and the connectors is usually enough to restore contact. Reconnect firmly until each pin clicks in.
This is also worth checking if the glass has been replaced recently - the connector can look connected without being fully seated, which gives the same result as no connection at all.
Cause 4 - A broken ground wire
The heating circuit needs both a power feed and a ground to work. The ground wire runs through the door loom alongside the other mirror wiring, and on some cars it passes close to the door hinge - a section of wiring that flexes every time the door opens and closes. Over years of use, that repeated flexing can cause the wire to fatigue and break internally, even with no visible damage to the outer sheath.
This one takes a multimeter to diagnose properly. With the mirror heating switched on, test for voltage at the connector pins on the back of the glass. You should read around 12 volts. If you're reading close to zero, there's no power reaching the mirror - either the fuse, the switch or a break in the feed wire. If you're reading 12 volts but the mirror still doesn't heat, the ground is likely the issue.
A broken ground wire can sometimes be found by running a temporary earth from the connector pin directly to a metal part of the car body. If the mirror heats with the temporary earth, the original ground wire has failed and needs tracing and repairing.
Cause 5 - Failed heating film in the glass
If everything above checks out - correct switch position, fuse intact, connectors clean and seated, voltage confirmed at the pins - the heating element inside the glass has failed. The film is a thin resistive layer bonded to the back of the glass, and it can crack from impact, age or thermal cycling. Once it breaks the circuit is open and no heat gets through.
You can confirm it with a multimeter set to resistance (ohms). Touch the probes to the two connector pins on the back of the glass. A working element shows a resistance reading - typically 1 to 10 ohms. No reading at all means the film is broken.
The film can't be repaired - the whole glass needs replacing. Since it's a clip-on fit, that's a two to five minute job at home. Make sure you order a heated replacement, as it has the connector pins on the backing plate that a non-heated glass doesn't. Browse by make and model in the wing mirrors collection.
Fitting instructions are in the wing mirror glass fitting guide.
One more thing - temperature-controlled heating
Some modern cars regulate mirror heating based on the outside temperature. If it's above around 16°C, the system reduces power to the mirrors or turns them off entirely on the basis that they don't need it. This is normal behaviour on a number of Land Rover, VAG and other models - but it can look like a fault if you're not aware of it.
If your mirrors seem to work fine in winter but don't heat in milder weather, this is likely the explanation rather than a fault. Test on a genuinely cold morning before assuming something is wrong.
Common questions
Only one of my mirrors has stopped heating - where do I start?
If one mirror still heats normally, the fuse is almost certainly fine - both mirrors share the same fuse on most cars. Start with the connector pins on the non-working mirror, then check for voltage at the pins with a multimeter. If you're getting 12 volts but no heat, it's the element in that glass.
My rear window heats fine but neither mirror does - why?
They often share the same switch but run through separate circuits or fuses. Check the mirror-specific fuse first. On VAG cars in particular, also check that the mirror knob is in the heating position - the rear demist alone won't activate the mirrors on those models.
Can I repair the heating film myself?
Heated rear window repair kits that use conductive paint do exist, and in theory the same approach could work on mirror glass. In practice the heating film in a mirror is much finer than the elements in a rear screen, and the repair rarely holds reliably. Replacing the glass is more straightforward and cheaper long-term.
The mirror takes a long time to clear but eventually works - is that a problem?
Slow heating usually means a partial break in the element - the circuit is intact but the resistance has changed. It'll clear condensation in mild conditions but won't deal with ice effectively. It tends to fail completely before long, so it's worth replacing the glass before it becomes an issue on a cold morning.