Why do you want to do this? Most people go the other way - removing the pressure switch and putting in an actual switch. If the diaphragm in the pressure switch fails you loose your brakes.....
To answe your question, your wires that went to the manual switch will all go to one prong of the diaphragm switch, the other prong gets grounded.
So the two wires that were connected to the one terminal they go on one and then the other one gets grounded is that a ground wire that comes off the pigtail of the manual
