I just built a Volvo 240 a couple of days ago, and your problem sounds familiar. I think part 1 turned out to be an antenna of the microscopic type, just a tiny "pimple" on the rear fender. My car was unwashed so quite yellowish and the tiny outline of the antenna when in mount mode was almost the same colour as the car.

There was also another car (maybe I got them mixed around) where an illusive part turned out to be a "spoiler delete", that had to be placed on the INSIDE of the bootlid/trunk, so if you can't find the antenna, maybe try checking the inside of the trunk.

Just checked, and it IS the "spoiler delete", visually represented by a yellow sticker on the inside of the bootlid/trunk. You can't see it unless you open it.

Thanks it was a sticker on the inside of trunk - posted a screen shot on my profile as a reminder. Car now completed.

thx for this post it did my head in looking for it,a sticker on the inside of the trunk how the hell come up with that.