To help us understand, please try to be specific with your replies. The above reply may seem obvious, but it is open to two interpretations. No lights at all, or lights come on and do not dim.
Start at the beginning. Measure voltage across starter battery terminals under cab floor.
Is earth connection in good condition, and firmly attached? Likewise is positive fuseboard securely attached, (Do not use spanners on the positive without taking precautions. Ask for further advice if needed.
Turn on ignition. Does the battery voltage change?
The engine block earth strap is a weak link on x250 & x290 models. I would expect it to cause spurious alarms rather than sudden failure. Has it been replaced recently?