Also... I spent a few hours this afternoon removing the windshield washer reservoir, mostly to take a look at how feasible it might be to use this for up to 50/50 mix.
This is the location on the M2 where an F80 owner installed their pump. It's behind the front bumper cover, beside one of the auxiliary coolers. The F80 owner removed / relocated the horn and I believe used the bracket to mount the pump and ran the lines behind the arch liners from the windshield washer reservoir:
So here is the windshield washer reservoir removed. When you remove it, you don't need to remove the pumps (1 for headlight washers, one for windshield), but you do need to disconnect the wiring for the pumps and the level sensor. The image below is with everything disconnected and removed:
In this image, you can see the filler hole (cap removed) and a nice vent with a very fine mesh filter in the center of the tank at the top. I can't imagine any fluid would slosh out of this vent, and it's pretty much impossible for it to slosh out of the filler hole, but as others have pointed out... There is no way to know how much fluid is in the reservoir, and if you over-fill, it will pour out of the vent, down your inner fender and out the bottom of the rocker panel:
Here you can see the two holes pre-drilled in the tank for the windshield pump (smaller, lower hole) and the headlight washers (larger, upper hole):
The headlight washer hole might be great to use for a tank tap. It's not quite 22mm, but nearly. The issue is that it's pretty high up in the tank (as I'll show later), and you'd get almost nothing, if any warning if you were to use that point:
Here are a few other locations where a 22mm hole might work. I'm going to cut out some 22mm circles before I drill any holes to make sure the location I choose is going to work
Spare location for a washer pump:
Very bottom of the tank, at the front. I've seen an N20 F20 owner use this location before:
Slightly higher up, though still below the level sensor, basically where the spare washer pump would fit:
Next I plugged all the holes, and filled the tank slowly, making markings every 500ml to see the capacity of the tank, when the low level warning would be likely to come on, and how much would be left at that point.
As you can see, the tank holds 4.5 litres. I could probably get another few hundred ml in, but 4.5 is pretty much the max.
The level sensor (on the other side) is just below the 1 litre mark. I validated this later once I re-fitted everything and found that the low level warning went off if there was 1 litre in the tank. I guess it would be fair to say that there is at least 750ml of fluid left in the tank when the warning comes on, which is nice!
As I said before, I'm almost certainly going with a trunk mount tank, and have ordered one to do that. I did this work today because I wanted to tinker with the car, and I wanted to judge the feasibility of a windshield washer tank install.