Hello all -
Just closing the loop on this. I replaced both rear bushings and the thunk in much improved -Its not totally gone, but its reduced to where its certainly livable and if wasn't specifically driving over large bumps at speed and listening for it - you might, maybe, not notice. I decided to use the thicker 114 part number even though the ones I took out were the 115.
Removing the seat was easy - but probably took three times as long to put it back in. The rear seat is a bit fiddly - was was working by myself. I'd suggest starting the middle bolt first when reinstalling, then doing either side. Overall it probably took a few hours start to finish - and at least 30 of that was putting the rear seat back on ....another 5 was removing the bottom seat again because I forgot to reconnect the heated seat

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