Plastic pattress and earthing?

I have to extend my light switch wires from my original metal wall patress finishing in a plastic dry wall patress attached to my new fitted bedroom furniture, which has a 50mm cavity between the inner and outer walls of the wall unit. The earth wire is fitted to the dimmer unit but there is no earth terminal in the plastic patress. Is it detrimental to omit the earth, not from the dimmer but an earth wire back to say the original metal patress.