I think you can pry the plastic retainers out a little from the front, and then the backside will compress and the whole rivet-assembly will be able to slip right through the hole. When the pin is pushed in it causes the backside to expand so the resistor plate doesn't fall out of the heater box. Use a razor blade or a very this screwdriver to pull the rivets forward a little until the whole thing can be removed.
Push the center part of the pin out (toward the resistor) Wiggle it through the four prongs in the second picture. The pin will then detach. Squeeze the prongs together and push it out of the box. When you insert the pin it splays or forces the prongs out, holding it in place. Be careful and reuse them as they are not reproduced (to my knowledge). I broke mine when I rebuilt my heater box and had to use small black machine screws and nuts. Also, put a little strip caulk around the hole and sandwich it between the resistor when you reassemble it foran airtight seal.