WAY 1 Using Phono (RCA) leads
To remove clicking/poping noise when stereo mutes when aftermarket Head unit fitted.
Depending on Bose upgrade you will have i believe 2 or 4 phono plugs depending on how many speakers are amped up.
so you need to make up the same number of leads which are put inline with the new Head unit with phono(RCA) outputs.
see pictures for building

Tools requied

soldering iron
pair wire cutters
electrical insulation tape
pliers (with an elastic band on the handle to hold plugs so you dont get burnt)
screened cable
2 or 4x phono sockets
2 or 4x phono plugs (i used angled to get stereo to go in further and to fit in capacitors)
15uf ish capacitors in line pref

It seems to work well in my A6 with all bose speakers amped and sub
but due to bose EQ in amps you may need to reduce treable

Way 2 using speaker outlets with a Speaker to Phono(rca) line level reducer from such companys as autoleads etc