I do not have problem with Valet... yet. Problem is that master key is cut into the trunk lock. I can not take the part of the key away. No problem yet.. it has been like that for a while and all works fine until somebody will press valet button. Therefore I would like to know, is it possible to disconnect the whole function? It is so unnecessary function. Does it work if I just unplug the button or do I get some error codes then?

I have XJ8 1998.

 

No reason you shouldn't be able to unplug it. It is a Normal Open Circuit switch, pushing the button closes the circuit. Removing the plug would still be an open circuit.