[mcguyver]

The little "fingers" on the switch that contact the selector can be problematic at times. With some guns, they make poor contact, and if even 1 of them doesn't make contact, the circuit is open and the gun won't run.

Putting the mechbox back into the body could add some pressure to the selector plate, and cause 1 finger to not make contact.

Those fingers are the electrical component of the safety system, so in safe mode neither should make contact. I had an issue a few years ago where there was some heat generated at this junction (upgraded mechbox = more motor current = heat at poor junction), and it melted the plastic a bit where the copper contact on the selector plate was pressed. The little bit of flash would push out the lower finger, and the gun would only run in semi, no full. A hobby knife cleaned it up nicely.