After some thought, I decided that the easiest option was to utilise the washer bottle unit for the supply for the improved water spray unit I'm working on.
I'm not sure if it is Group A specific, but at the very bottom of the reservoir is a separate unit which feeds the cold start water injector. It's capacity is pretty small, so I decided to open it up to the main unit and install a level sensor at the same time.

Here is the level sensor taken from a water spray bottle used on a Subaru STi...



I started by making a circular hole at the very bottom of the reservoir to suit the sender unit and then cut a square hole (easier to make) between the two sections...



Once done, the unit was flushed out thoroughly to remove any plastic swarf. Don't want to jam a pump or block a spray nozzle! The seperate reservoir is clearly evident in the pic below.
The sealing grommet was then put in place..



... followed by the sensor itself...



A view from the top...



Plan is to utilize the OE cold start water pump which will be triggered via a relay by the Cusco water spray controller. This in turn will be boost pressure activated. I'll write more when I get to that stage.
Gary

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1994 Toyota Celica ST205 GT-Four Group A WRC - running in rebuilt engine....

Very nice work Gary, Looking forward to the finished setup, Im trying to setup the same thing using the Cusco Water spray Controller, The 91 Group A's came with a water spray as standard, but apparentley they rush out a bit too much water.

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As did the St205, but again, the nozzles aren't that flash. I'm going to use 2 or 3 WI nozzles which put out a very fine mist of water and should be ideal. Having an intermittent on/off spray will only help improve the effects of the spray - otherwise the rad has the potential to become 'waterlogged'.
Gary

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1994 Toyota Celica ST205 GT-Four Group A WRC - running in rebuilt engine....

Would having too much water be better than having no water at all though?

Throw too much water at the rad and it'll become waterlogged to a certain extent and there won't be an effective transfer of heat from teh rad to the water that's been 'chucked' at it. Idea is for a fine spray of mist which will evapourate quickly, removing heat at the same time. Otherwise it's a bit of a pointless exercise.
Gary

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1994 Toyota Celica ST205 GT-Four Group A WRC - running in rebuilt engine....

oh ok. so the standard nozzles are useless and don't help cool at all?
except when it's a 34-degree scorcher

Didn't say the standard nozzles are useless, just nowhere near as good as they'd need to be for an effective system.
Gary

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1994 Toyota Celica ST205 GT-Four Group A WRC - running in rebuilt engine....

yeah, useless is a bit of a strong word