u should have a pile of parts that look like this there should be a brass nut in the rubber grommet which is actually an expansion nut if there is not the rubber has rotted and the nut fell on the floor on dis-assembly and you need to run to the local ace hardware store and get two new .80 $ expansion nuts, the nylon washer is there to keep the bolt from grabbing traction on the rubber expansion nut and twisting it till it tears and fails. (possibly what happened to yours)
you assemble them like this then push it through the the plastic pannel and metal bracket till the flair on the expansion nut stops you, you then tighten the screw till it becomes snug not torqued down tight.
Now pretend the white washer with the line is your plastic and metal pieces and see how the expansion nut has expanded and will hold the two together.
hope this helps.