Yep. Could be the cap, or the wiring to it. I'm sure there would be someway to test it, but I don't know it. Try a friend with the same control lights, doesn't have to be the same car. Just wire the cap in, and slide the plunger up and down and see if the light goes out.
The plunger/sensor on mine is nackered, so for the time beeing I have a shot peice of wire, a spade connector on each end and pushed into the two wires, light has gone out. Its a quick fix, just keep an eye on the engine temp and regulaly check the water level