Here is a schematic of the Spider bumper assembly.... looks identical to the Coupe.
The bumper bracket, one each side (item 30) is bolted to the bumper itself with 2x 34 & 22 (Bolt and washer).
Then the brackets are bolted to the chassis through the two holes using item 16, in the picture the holes are not numbered but one is right of item 32 which is a rubber grommet for the rear license plate lights. Those wires pass through the small hole in the centre of the rear panel, directly above the number 16.
Hope that helps