INitially it was stuck on. a few months ago (I posted then to general "don't bother with rinse aid" advice :) )
Then it went the other way, and hasn't come on for months.
When you power on the machine, it flashes all lights, so it's not a question of the light/bulb having failed.
Dismantled front of machine - very easy. Removed rinse-aid/tablet holder assembly.
Inspected, and found an open circuit in the 2-wire cable between the rinse aid dispenser and the main board.
Replaced wires - tested from both ends - all OK.
Reassemble and (have you guessed ?!) - the rinse aid warning still doesn't work ....
Given the control module has (as I explained to SWMBO) "electronics up the wazoo" - is it likely there is some logic with some other components which is (also) failing ?
Just shorting the two wires by hand doesn't trigger the light. Door open. Door closed (in case there's an "ignore if door open" logic).
I think we'll get by, but it is intriguing ....