I need this line output converter for my setup, right?

I want to make sure I am doing everything properly. I have removed my head unit to check out the connections on the back, and there are no low level outputs.



This means I will somehow have to use the speaker wire going to one (or two?) of the rear speaker(s).



From reading around, it looks like i am going to need this:

http://www.monoprice.com/products/product.asp?c_id=108%26amp;cp_id=10844%26amp;cs_id=1084401%26amp;p_id=6908%26amp;seq=1%26amp;format=2



That's correct, right?



Now if so, my second question is:

Since my minivan's rear speakers say 30 watts on them, and all head units are limited to 20 watts by the battery power, I am going to assume the head unit is sending them 20 watts.



Now, with 20 watts going through the line output converter (its rated for fifty so that will be okay), but, won't the 20 watts going into the back of my sub amplifier hurt the amp? If not, where do those 20 watts go to?I need this line output converter for my setup, right?
well the power varies by the deck and yes you need the line converter the line converter will use its internal resisitors to consume the 20 watts as heat