Looking at this circuit diagram there appears to be two under the dash associated with the hazard lights. The low brake fluid warning relay appears to be the same on the drawing so you maybe could swap it over in the absence of a spare, the wiring colours should identify the relays once you are in there amongst it.
There is a multiway connecter in there too ? Another one associated with the hazard switch ?
www.gomog.com/allmorgan/4~4%20Diagram%20for%20Morgan%20Dash%201800%20Focus.pdf
Lifted off the gomog site 1998 - 2004 4/4.
Good luck.
