I'm installing Infinity Kappa 6.5 in. componets in my doors. Can I tie into the speaker wire behind the HU or should I just run new wire from the amp through the doors to the speakers? It seems like a PITA to run the wire through the doors. Will 18 gauge suffice for the componets? I'm using 12 gauge for the subs.

 

Run new wire from the amp.

 

It is a PITA to run wire to the doors but you need to.
And I would use 16 gauge. Twelve gauge is fine for subs.

 

I have the Infinity Kappa perfect 6 1/2" and i ran 12gauge to the door speakers and 8gauge to the subs(perfects also). Depending on the amp being used i would go for at least 16-14 gauge and run it to the doors since factiry wireing sucks if you ask me