The rear subframe bushings are quite difficult to replace. The subframe has to be dropped from the chassis because the forward pair of bushings has to be removed and replaced from above. The rearward bushings can be replaced from below (subframe can stay in place). I'm not a DIY kind of guy but from reading this forum and talking to my indie shop, this is a job for experienced mechanics only. The job is usually quoted as taking anywhere from 4 to 8 hours of labor. You also need a special tool to press in the new bushings (not sure about removing the old bushings).
Best of luck.
NEW: 2013 128 Coupe / 6MT / Premium / M-Sport / Cold / Xenon / HK Audio / CDV Delete
OLD: 2009 128 Coupe / 6MT / Premium / Sport / Cold / Xenon / HK Audio / CDV Delete / M3 Suspension