Audio help?

I've been out of the loop on car audio for many years. I know enough to install a deck and that's it. My new 99 Rodeo came with a sub-woofer and what appears to be amp power wires running to the back but no amp. Is the amp what drives the sub-woofer? If I wanted to make the sub work, what do I need for an amp? I don't want heart-stopping booming, just enough bass to sound good, not rattle the windows in the neighborhood. I'm thinking some kind of small-ish amp would do the trick with some sort of low-pass filter.

Yep, you got it, an amp with a sub-output with enough power to drive the sub. You'll have better sound if your CD player/deck has RCA outputs and your amp has RCA inputs.

How big of an amp would I need to get some decent bass? Again, not huge bass but just good sounding bass that sounds good INSIDE the truck.

Look at your sub and see how many watts it's rated for, then, double that for clean sounding bass that won't rattle the neighborhood or your truck to pieces.

Generally, the heavier the amp, the better the quality. Don't buy an amp that is rated for 1000 watts and weighs less than the box it comes in.