Don't worry ,go ahead with the wiring with your desired unit.Who said by using the coupler harness arrangement warranty will be voided.
Make sure they do the work neately without tampering stock wiring ,and make sure the fuse is also connected properly.
When i asked whether i could fit rockford fosgate sub woffer on my ride,the accesory store guys of maruti told rockford will void warranty ,only JBL,Sony are allowed and that too from them.
I asked how the brand is related with warranty
They told work done at ASS
only will be regarded as warranty proof.
I also told what if any issues crop up with wiring at your store,they didn't have a reply nor did they give it was 100% perfect.
I did all my accesories from outside,and car is running fine.
I add that mechanical modifications will void warranty for sure,but electrical ones by using the specified fuse and other components and coupler wiring no issues will come.