Do you have a 24V system or a single battery?
I looked at one recently and found a second battery hidden away under the tub which, at one time, had been used to run a 24V ex-MoD radio!
If you only have one battery, check the voltage going to the lamp and then trace it back using a meter and the wiring colours. Once you have found out where it is fed from, and assuming that it is surplus to requirements, you should be able to isolate it at the source end.