A great way to implement a low fluid lamp is described in this Joe Perez thread:

https://www.miataturbo.net/methanol-water-injection-22/different-sort-water-tank-setup-9256/

The gist is that the OEM cluster of Joe's 1992 provisioned a low fluid lamp. This is shown in the following pictures lifted from his thread:





Thinking that this was slicker than snot, I dug into my 1990. Alas, this is what I found:





I'm not sure when the changeover happened. However, you can easily see which cluster has the extra lamp positions from the front. You should check this before tearing into your dash.

Now, I'll have to do something not nearly as slick.