It could be the switch that operates this warning lamp has gone duff... it happened on my cherokee diesal....
The best thing of course is to replace the switch , it plugs into your plastic water bottle...
Or.... you can do what i did , and pull the switch out , disconect the wires , and join the wires together....puts out the annoying light , but of course you then have no warning of low water level....
My diesal is running on steel seal too....done thousands of miles with no problems... good luck....