I wish you'd make up your mind...
first of all the stock amp is a piece of crap, if you're replacing the speakers, then get an aftermarket amp because they will sound terrible otherwise.
If you get an amp with high level inputs you should be able to just plug in the speaker leads to that, no need for RCAs. I've never used them but that should work.
If that doesnt work you can buy line converters that make the speaker leads RCA's but they're expensive for decent ones
IMO you'll be much happier if you take out the stock stereo and run all new wiring. Takes 3 or 4 hours and you'll be much happier with the sound, I was.
Here's a great article on adding an amp to a stock deck and your options
HERE
the aftermarket deck wont look as bad as you think. I sanded down the face of the dash kit and painted it flat black to match the A/C controls. Matches perfect, have a look
This is the last you'll hear from me on this and I hope to god that anyone else planning to do this, finds this thread
oh and I'm pretty sure the wiring is 16 guage