Open the tailgate and look towards the top above the glass. There is a long black rectangular cover panel. Pry that cover off to expose the hoses and fittings.
We have had more than a few cases where the check valve clogs with an unidentified white substance. Looks like fine ground white plastic.

 

have a 2018 xc60 it has been in the Volvo work shop three time for the same problem and they can not fix it . I want them to remove washer tank and flush it out but they said this is not the problem. We will just flush out system, I told them that this will not remove white stuff from the tank as the car is stationary white stuff will remove around when car goes over any bumps and will be pumped out onto car if wiper is used . They just said rubbish.

 

As a matter of possible issue prevention I run the back washer at least once a month. In the dry months right before washing the car. I’ve had rear washers plug up before and this seems to help. I’ve come across that white powder as well.

 

I tried my out yesterday and the same problem.. Today I looked at the Volvo XC60 rear washer parts sheet 2018 Volvo XC60 Washer equipment, rear. I took off from the nozzle #1 to the quick coupling #8. I separated all the parts and cleaned out with either compressed air and Calcium cleaner that you use for clogged bathroom shower heads.
When reassembling make sure black coupling part # 5 is installed correctly. This has a check valve in it. It should be installed not to allow the fluid to go backwards.

Fluid flows out like new. Probably saved over $100 if I brought it in to Volvo.

 

I tried my out yesterday and the same problem.. Today I looked at the Volvo XC60 rear washer parts sheet 2018 Volvo XC60 Washer equipment, rear. I took off from the nozzle #1 to the quick coupling #8. I separated all the parts and cleaned out with either compressed air and Calcium cleaner that you use for clogged bathroom shower heads.
When reassembling make sure black coupling part # 5 is installed correctly. This has a check valve in it. It should be installed not to allow the fluid to go backwards.

Fluid flows out like new. Probably saved over $100 if I brought it in to Volvo.

Is there any trick to prying the cover off? I'd hate to break it.

 

Stumbled upon this forum with the same issue. Front wipers/fluid are fine but the back windshield will NOT spray the glass at all (wiper moves just fine). Well, after removing the black cover as suggested and popping-off the clip that joins the main hose end to the smaller hose end that runs to the windshield nozzle, I was able to test flow from the front to the back as fluid ran freely out the end of the disconnected main hose. So, that left the smaller "pig tail" hose that connects to the output nozzle/sprayer on the exterior of the windshield. To get that thing out, you somehow reach into the small metal space where it connects inside the body, and pinch both sides (you'll feel the pinchers) to push up and out. It'll eventually get loose and push out of the exterior of the car. Small hands can probably do this easier!!

After running a blow-dryer along all the connector pieces to loosen the rubber, I was able to dismantle all of them and found the culprit! A HUGE chunk of fluid/plastic/calcium had lodged into the last hose connector and it was a full-on blockage. It was actually hard to get out! Removed the blockage and then dunked all the parts in a cup of CLR to get rid of anything that might still be caked in there. Reassembled and waaaaaaaah-laaaaaaaaaaaaaaaah, a working back windshield wiper for the first time since we purchased our pre-owned beauty! Hope this helps others that may stumble upon this issue! Maybe I should go to work for Volvo from the sound of some dealerships!

 

Just for help in case someone wants to take care of this. The check valve sucks and costs 18 ish usd.

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This thread has been very valuable to us. The XC60 is not my car, it belongs to my wife (but I wrench on my 10 and 25 year old Benzes). Following the directions above, I tried to clean out the check valve and quick coupling. The check valve cleanup may have been successful, but the quick coupling contained a tiny washer/gasket that either disintegrated or fell out, so it leaked like a sieve after just one cycle of coupling. Maybe it got "blown out" when I was clearing the white powder from the system - so, look out.

I got the parts at the dealership (a dual barb, the one-way valve, and a new quick coupling) for about $55 USD. There was nothing wrong with the dual barb piece but this took two days to get here (not in stock in dealership) and I didn't want to take any chances for $10. Worthy of note is that the quick coupling AND the one-way valve originally in our 2018 Volvo have been superseded with new part designs. The quick coupling is now white and seems sturdier, and the one-way check valve now has a much bigger "check valve" with a convenient arrow on it.

We never put low quality windshield washer fluid in our cars (or just water). But I believe the combination of not using the rear washer AND parking outside with the tailgate facing the setting summer sun may contribute to the evaporation of the fluid, leaving the "soap" from the fluid (I doubt that the white powder is plastic or calcium). So, I would DEFINITELY operate the rear washer at least once a month, possibly more during the summer particularly if you live in a hot climate.

CAUTION: When removing the parts and then operating the washer to unclog, put down a towel first because that white soap is hard to get out of the carpet, netting, and so on.

 

Just did this. The check valve was clogged, so I tossed it into my ultrasonic cleaner and it dislodged most of the sludge.
Thanks gang!