I would check that the fuse that blows actually protects the stereo and reversing lights. My 156 is a 1998 so may be different to yours, but according to my user manual, the reversing light fuse is shared with the high level stoplight, electric windows, central locking and wing mirrors (fuse 13). The stereo is grouped with the alarm and instrument cluster (fuse 12). Do you have a user manual for your specific 156?
In general terms, if a fuse blows it means one of the following:
a) the fuse fitted is an incorrect value (i.e. too low)
b) one of the items protected by the fuse is faulty
c) the wiring connecting the fuse to the protected items is faulty
Start by verifying exactly what the fuse protects and check that it is the correct value. Don't be tempted to just fit a higher value without checking the manual as it is possible to start vehicle fires in this way.