How do you get the passenger backrest pad off of the sissy bar?

I would checkout Sissy Bars, they have exactly what you are looking for and have guides on installation

Probably ... the trunk will come with back pad ... you will not need the luggage rack or the sissy bar any longer.

Thanks for your visit today.

the bolts on the rear fender area hold the sissy bar as well as the rear fender. I had the same difficulties. you will have to buy long bolts that will fit through the sissy bar and through the chrome and then through the fender. I found this out the hard way when I removed the sissy bar and did not replace the bolts. the rear fender was close to falling off the bike.
the bolts are found in diy stores. the only restriction is to make sure the bolt does not go into the area of the rear wheel so that it might come in contact with the tyre.

As per what Sev first said.... check all nuts and bolts. Not just the seat/sissy bar mounts, all the engine mounts, swingarm pivots, wheels, calipers, covers, bars, pegs, clamps.... everything. But most obviously the engine mounts, a loose or busted engine mount can make all sorts of odd things vibrate. While you're checking them out, make sure that any mounting plates or braces are not cracked, and that the bolts haven't stretched and are being torqued against their own threads, instead of the mounting hardware.