I
As I said, I don't have any terms and conditions; it is a service contract listing all the parts to be installed by us. There is no appendix with terms and conditions nor any other reference in the contract.
Then you have a work contract according to the Building Code or VOB; it should be stated there.
I’m more concerned about my negotiating position
A stubborn person (I want, I want) is always in a worse negotiating position than someone who makes the other an offer, suggesting that they are the winner in the situation, even if they are not. That is called diplomacy and negotiation strategy... you have to weave in the hint about the electrical work yourself. Maybe can give you a tip in that regard :)