I was speaking to a mate recently who also has an ST (although I doubt this is an ST-specific issue), and he advised me to check the rubber grommet where the rear wiper spindle comes through the boot. Sure enough, mine had split too. I'd suggest anyone with a Mk7 Fiesta that's more than 3 years old (mine's a 2016) checks theirs too, as water going in that hole will get inside the boot and into the boot electrics.

Replacing the grommet is simple enough, but does require stripping down quite a lot of the boot. The part you need is 1701120, available on eBay (through Rates Ford) for £4.99:

https://www.ebay.co.uk/itm/Genuine-Ford-Rear-Wiper-Spindle-Grommet-1701120/272377981252

You'll also need 8mm, 10mm and 13mm sockets, and a T20 torx driver (the 8mm can be a spanner if your socket set doesnt go that small).

1. Lift up the cap on the wiper and remove the 13mm nut and washer that holds it on the spindle
2. Pull the wiper arm off
3. Open the boot, remove the parcel shelf and undo the three T20 torx screws on the trim next to the window and inside the interior handle recess
4. Pull off the side trims next to the window, being careful not to break or lose the clip holding each on
5. Pull off the rest of the trim, again being careful not to break or lose the clips (of which there are many)
6. Unplug the wiper motor electrical connector, undo the three 8mm bolts and washers holding it on and remove
7. Undo the right-most wiper motor bracket bolt (I forget if this is an 8mm or a 10mm, but this is so you can access one of the nuts for the next step)
8. Undo the five 10mm nuts holding on the exterior panel that houses the number plate lights and boot release button
9. Carefully unclip the exterior panel and let it hang on it's electrics (unfortunately this can't be unplugged from the loom)
10. You can now remove the old grommet, give the area a quick clean and fit the new one from the outside (which comes pre-greased to accept the wiper motor spindle)

Refitting everything is the reverse of the instructions above.

Hopefully this will help someone with what seems to be quite a common issue 

I didn't know any ST's still had their rear wiper...thought they all had the 'Kill All Wipers' bung with a white sticker of confirmation around it to ruin the clean aesthetic that they achieved by removing the rear wiper in the first place...  

Good write up though, it should probably be moved to the guides section.  

I did actually do some research into what was involved before I started by watching some youtube rear wiper delete videos!

Thanks for posting the link to the seal. Mine split a while back and I was not able to get one from the local dealer so ended up using Tiger seal on it to re seal it a bit of a bodge but i had to stop the water getting into my boot lid every time it rained. Anyway thanks again for sharing.

Just noticed mine has split, I searched on eBay first and found the part, then a Google search of that part number and I've ended up here along with instructions and matching part number.

You can't get better than that, thanks

I saw this and thought to check mine and it is cracked. 
 

Seems it’s quite common then 

On 7/4/2020 at 1:32 PM, TimST3 said:

I was speaking to a mate recently who also has an ST (although I doubt this is an ST-specific issue), and he advised me to check the rubber grommet where the rear wiper spindle comes through the boot. Sure enough, mine had split too. I'd suggest anyone with a Mk7 Fiesta that's more than 3 years old (mine's a 2016) checks theirs too, as water going in that hole will get inside the boot and into the boot electrics.

Replacing the grommet is simple enough, but does require stripping down quite a lot of the boot. The part you need is 1701120, available on eBay (through Rates Ford) for £4.99:

https://www.ebay.co.uk/itm/Genuine-Ford-Rear-Wiper-Spindle-Grommet-1701120/272377981252

You'll also need 8mm, 10mm and 13mm sockets, and a T20 torx driver (the 8mm can be a spanner if your socket set doesnt go that small).

1. Lift up the cap on the wiper and remove the 13mm nut and washer that holds it on the spindle
2. Pull the wiper arm off
3. Open the boot, remove the parcel shelf and undo the three T20 torx screws on the trim next to the window and inside the interior handle recess
4. Pull off the side trims next to the window, being careful not to break or lose the clip holding each on
5. Pull off the rest of the trim, again being careful not to break or lose the clips (of which there are many)
6. Unplug the wiper motor electrical connector, undo the three 8mm bolts and washers holding it on and remove
7. Undo the right-most wiper motor bracket bolt (I forget if this is an 8mm or a 10mm, but this is so you can access one of the nuts for the next step)
8. Undo the five 10mm nuts holding on the exterior panel that houses the number plate lights and boot release button
9. Carefully unclip the exterior panel and let it hang on it's electrics (unfortunately this can't be unplugged from the loom)
10. You can now remove the old grommet, give the area a quick clean and fit the new one from the outside (which comes pre-greased to accept the wiper motor spindle)

Refitting everything is the reverse of the instructions above.

Hopefully this will help someone with what seems to be quite a common issue 

This, 5 years later, explains exactly what i need and am looking for

Many thanks TimST3!

 

I've a standard mk 7.5 but the wiper grommets are obvs of similar quality to the STs lol