Windscreen washer fluid level
Yes, it would have been more logical to combine the two reservoirs into one: as we generally use the rear window washer much less often than the front one, its reservoir is usually still nearly full when that of the windshield is already empty. You can improve the situation by connecting the two reservoirs together. Drill a hole into the sidewall of the reservoirs near the bottom. Fit a banana jack into this hole (don't forget to use some sealing gasket or silicone sealant before you secure it with a nut). Fit a plastic tube over the jacks in the two reservoir to form two communicating vessels. If possible, use a tube with rigid walls to avoid it being clamped under liquid bottles or any other item stored in the bulkhead cavity.
As the reservoir capacity will be practically doubled, it would be a good idea to add a level warning as well. National Semiconductors manufactures a specialized integrated circuit (LM 1830) designed for fluid detection systems.
This circuit can be used to measure the level of conductive fluids. Chemically pure water is not conductive but tap water (especially if mixed with windshield cleaning fluid) is. It is recommended to use a LED as indicator, in this case there is no need for any additional part (using a bulb would require an additional capacitor between pin 9 and the ground).
In most cases you will be able to find a warning symbol on your instrument panel which is not actually used (manual choke, ABS or engine malfunction warning, catalytic converter warning; mostly in the corners of the tachometer). If everything else is used up, you can still think about the front brake pads warning lamp—after a few years this warning is not too reliable anyway, so you might decide to use it for another purpose. Although, as mentioned before, you could use a standard bulb, too, the best solution is to solder a white (uncolored) LED into the bulb holder. As the LED is sensitive to polarity, you will have to note the way you place the bulb holder into the instrument panel.
You will also need a probe in the reservoir. The easiest solution is to use a simple jack phono plug. Use a plastic tube with the appropriate diameter, glue the phono plug into its end using a silicon sealant. Fix the probe into a hole drilled on the top of the reservoir so that its tip reaches down to the level required. Once the fluid level drops below the tip of the probe, it no longer connects the two connectors of the probe and the integrated circuit lights up the indicator attached.
The rest of the circuit can be housed in a small plastic box and placed either at the end of the probe tube on top of the reservoir or fixed to the bulkhead cavity wall somewhere behind the reservoir. The wiring can be routed together with the wiring harness of the washer pumps crossing the bulkhead into the cabin. The circuit should be fed from a 12 V switched by the ignition key so that it will remain silent when the car is not running—or better yet, from the wire feeding the washer pump: then it will only light up when you try to use the washer.