1985 Mercury Marquis starts but won't run

If the mechanic checked the switch continuity, and found the ignition switch to be no good, then thats where you start! It's a white plastic box that the wiring harness plugs into, and it's mounted on the steering column with two small 5/16th screws. Remove the negative battery cable, if it has air bags, wait two minutes then remove the plug, then the screws, then pull it out...install it the same way, then put the battery cable back on. You will need to remove the screws that hold the column covers on, then you will see the switch on top potion of the column.

Pay attention to the rod that moves the switch, it goes into the switch only one way!