18 gauge speaker wire for 100 watts?

I am installing my alpine pdx amp into my 2011 WRX and don not want to go through the hassel of running wiring through the doors like on my speed3.  I found the metra quick wire 18 gauge speaker wire bundle and want to run that from the PDX amp back to the original wiring behind the dash.  Here ia a pic.

Will this degrade the quality of the sound at all?

I'm pretty sure most speaker wire ranges in thickness from 12-gauge to 16-gauge. It depends on the quality of the wire, how long the wire needs to be etc... Longer runs of wire can cause power loss and then require a heavier wire.

Myself, I NEVER use anything besides 10g or 12g speaker wires in my cars unless I'm running speakers off the head unit. I wouldn't use less than 10g even if my amp was 25 watts per channel. After all, wire is cheap (sometimes) and the time and effort to replace it is not worth the hassle. Use a heavier gauge to start with and you wn't need to worry. But in case you want the formula that is suggested in the MECP Study Guide, here it is (in part, assuming you're not going to be running longer than 10 feet of speaker wire):

(Read down in sections divided by commas)
Total Amperage Draw of System: 0-20, 20-35, 35-50, 65-85
Up to 4 feet: 14g, 12g, 12g, 6g
4 to 7 feet: 12g, 10g, 8g, 6g
7 to 10 feet: 10g, 8g, 8g, 4g

I highly doubt your amp will draw anywhere near those amperage levels. But my suggestion would be to run either 12g or 10g from amp to speaker, NEVER 16g or 14g.

This post was based partly on my own experience and opinion AND what MECP suggests.

With only 100 watts per channel, and the length of the run, even considering the oEM wiring, you will not be able to hear any degradation of your sound.

There ya go, OP, an "expert" answer straight from an idiot! LMAO! No, seriously, this last guy seems to really know his stuff. So if you don't take my advice, take his!