Who knows the gauge of the wires for the 4 x 6's and the 6 x 9's? I can't tell if it's 16 or 18, or 18 up front and 16 in the back?

They should both be 18 but to be sure, get a cheap wire guide/stripper and measure. 18 is fine for stock systems and most modified setups. Heavier wire will only be needed with a high output amp you may want to add. In that case, follow the recommendation of the amp manufacturer.

I measured it at 16 and brought a piece to Lowe's to double check. They got 16 too. Not that it matters in this case...I was thinking I would have to splice some wire onto the existing stock wire because of a bad cut, but there was enough extra from the leads from the new speaker to reach down to the stock wires.

Thanks though!