I just put a set of Piaggio rear crash bars on my GTS. I got them from Motorsport Scooters. (shameless plug ) The instructions that came with them was vague and incorrect. I thought I'd have some fun with my digital camera while I was putting them on. Sorry my bike's a little manky, but the roads have been yucky and I haven't gotten the hose out yet.
Remove the screw at the bottom front corner of the rear lower cowl.
The included instructions show removing a screw from the outside of the cowl, but there is none.
Remove the nut from the inside of the cowl, below the turn signal.
Gently pop the cowl from the body. It's held on by pegs that fit into the grommets in the body.
Here's the hole and the carriage bolt that hold the rear of the cowl onto the body.
Here's the carriage bolt
Remove the screw that was covered by the cowl. Also remove the filter cover, circled in yellow. Be gentle with the pegs, they're easily broken.
Trim away some of the leading edge of the filter cover. I used a Dremel tool.
Install the front bracket with the countersunk screw provided in the kit. The yellow line is where I trimmed the filter cover. This may not be necessary, but I remember someone cracking it on his GT, from the screw hitting it.
Put the cowl back on; tabs are on the front of the cowl and can be tricky to line up and get them back in. Install the rear bracket to the inside of the body, but do not tighten the nut, yet. This shot was taken from the opposite side of the bike to show how it goes on.
After repeating the procedure for the other side, bolt the two halves together. Tighten this bolt/nut first, then tighten the other four bolts/nuts holding the bars to the brackets and then tighten the nut inside the body.
Successful installation! Note that the US spec reflector stays in place.