Sorry for the delayed reply. Meant to reply yesterday evening but ended up getting side-tracked with other things.
As previously mentioned, the 12v socket, heater control unit, and the rear windscreen all share a single common power feed. These are the only 3 items on that particular power feed line so it's a fairly high probability that it is that common power line which is the cause of the issue you're experiencing.
The power line that supplies those 3 items runs from a single 50A fuse under the bonnet, through a relay which is inside the cabin, then into the internal fuse box where it provides 3 individual smaller fuses with power, one fuse to each of the 3 items.
As all 3 items are failing together, the most likely cause is that either the relay is faulty (or has a dodgy connection), or there is something a dodgy with the 50A fuse.
The first thing I would do is to re-seat both the relay and the fuse in case it is just a bad contact in one or both of those. To do this, you simply just need to unplug them and plug them back in again. And while they are out, it is worth checking for any oxidation on the spade connectors of the relay/fuse. If they don't look great then use a bit of scotchbrite to clean them up before placing them back into their respective sockets.
To find the relay in question, you need to remove the piece of trim under your light switch to the right of the steering wheel. This will then reveal a hole where your
OBD connector is located along with a couple of relay carriers. To remove that piece of trim, put your fingers around the bottom right hand edge and carefully yank it off. It's just held on with a few spring clips...
Inside the hole, you will see two relay carriers, an upper one and a lower one. It is the lower one you are interested in and that one will contain either 4 or 5 relays (the 5th one is only present if your car has engine start/stop functionality). The relay that controls the power circuit in question is the grey one on the left with the number
373 on which is closest to the
OBD socket and is in
relay position #2 on the diagram below. This is the relay that you need to re-seat (and also clean up if necessary)...
After you have done that, open the bonnet and locate the main fuse box. This is the box on the right hand side of the engine bay directly to the right of the battery (
location #2 on the diagram below)...
Once you've found it, remove the lid by sliding forward the two levers that are on the lid. Then locate fuse number
SB30 (marked as "F30" in the fuse box itself). This is the bottom most fuse on the left hand side that is directly above item "A2" in the diagram below. It should be red because that is the standard colour code used for 50A fuses. It is this fuse that you need to re-seat (and also clean up if necessary)...
Do the above as a first port of call and see if it resolves your
problem. Please report back with the results, whether it fixes your issue, or if you still experience the same
problem as before.
