Has it ever worked ?
Because, from what you describe "screw into a plastic fitting", are you sure you aren't suposed to have a gauge instead - as the sender for a gauge is a whole lot bigger (looks like an old camera film plastic tub almost, but white / orange / some other colour they had in stock when manufacturing lol)....whereas the sender for just an on/off bulb is relatively flat / small.
G
A picture is as good as a million words :
This is the on/off sender :
This is the oil temp sender, found in the same housing :
This is for an oil pressure gauge :
I guess, from your description, the first switch is kind of in a plastic housing, but that is only the spade connector which your wire connects to, but the actual item what screws into the block is metallic.
To test, turn your key on.... take the wire off your sender, and touch it to earth - your light should come on. If it does, your sender is kaput.
Hope that helps
Gordon
Edit again : thinking about it, if you had the sensor for a gauge then your light would probably be on dim then brightening up etc as the pressure changed.... so i think i was jumping the gun a bit.