[ericr]

you might well have mis-matched parts, looking at the plating on each. the lower section is less likely to get changed through ownership history, since you have to drag the whole wiring harness through it.

I have had the same difficulty and I have to sand the edge of the bottom piece very lightly to get it to insert into the upper piece, plating be damned. I also put a tiny bit of lubricant on that edge. At best it is a tight fit and I had to push/twist like hell to get it to fit and lock.

I also sanded the switch plate circumference on the end of the harness so it goes into the lower section easier and did not distort the part it fits into.

If I were you I would simply buy a repro of the entire switch, and get it to fit together while off the car.