you will need to remove the mirror to do this right. Search for a DIY on removing E9x door panel. In the E90, there are three torx screws holding the mirror in place behind a speaker (BE SURE TO HOLD THE MIRROR AS YOU REMOVE THE LAST TORX SCREW OR YOUR MIRROR WILL SLAM AGAINST YOUR DOOR). There will be wires still connected to the mirror but you can just place a towel on your paint and gently lay the mirror against it while you wrap and then just replace everything.
Cobb E40 PROtune by Jake@PTF/RR catless DPs/aFe DCI/ETS 5" FMIC/Alpina B3 TCU flash/BMW Perf. Exhaust./BMW Performance Style 313s/M-Tech Rear