I allow them to try. when they start asking questions, I tell them where the schematics are, and remind them that they own what they break. they usually pull wires and leave for my guys to figger out where they go. more and more, with things being what they are, they are becoming fraid to touch our stuff. statements like "hey, you know that board is worth fourteen grand" dont make them comfy
Just tell them to go ahead and do "their work", but make it clear to them that you will be billing them at your full service rate if you have to fix "their work".