If it's completely dead thats usually a good problem believe it or not
Usually it points to a power supply or rather the lack of it problem and can be an easy fix at times.
Check the power leads and connections to the unit for corrosion or any form of dirt if you have or can get your hands on a multimeter check that the voltage is at the pins of the connector if it nots thats your problem, also check that the contacts are good and clean in that fuse holder, if all is OK and you have power right at the unit check the socket the power lead plugs into for corrosion or any form of dirt. If you find any clean em up as best you can then apply some vaseline to all the connection and reassemble them.
If all was OK by now it's a problem within the unit itself
You could then try one of these:
Dekkaman Marine Ltd
Tramore Ind Est
Or Dumast : http://homepage.tinet.ie/~dunmast/index2.htm