Okay ,we're talking about the BOTTOM of the switch with the plastic plunger and the small center, metal pin... ... The neutral safty is operated by the big plastic plunger on the switch.. .For park and neutral this plastic piece drops into a recess in the "rooster comb"{this feature is basically a ground}.....For reverse, the recess in the comb is smaller...So the plastic neutral safty plunger just rides along the comb, but does not drop...But the small steel pin does drop into the hole, bottoming the steel pin in the plastic plunger case and completing the circuit for the back-up lights{the two outer wires on the switch}..
..So what happens when the pin is too long is:....When the pin drops into the recess, it doe'snt extend the full travel and can't bottom out in the plastic plunger because the pin itself is too long....filing the pin will allow it to bottom completely in the plastic neutral housing when in the rooster comb reverse detent{recess} and complete the circuit...

Here's what to try to see if you're on the right track....Take out the saftey switch from the transmission and hook up your wiring harness....You should have back up lights, if you don't , there's something else wrong{faulty switch or wiring problems}..To check for a faulty switch, run a jumper between the two outer wires on your back-up/ safty switch harness..If you have lights, the switch is bad....If still no lights you have a wiring problem ... . Now,assuming you have reverse lights... If you push the center pin in by itself the reverse lights will go off....It does'nt matter where in its travel the plastic neutral safty plunger is...All the way in or out or anywhere in the middle, if the pin is extended, the lights are on because ...the pin only has the opportunity to extend{by itself} out of the plastic plunger in the reverse recess on the rooster comb...
Does this make it any clearer on what COULD be the problem ???