Subwoofer in an 03 Tundra?

Based on a friend's 05 Tundra, the stock door speakers look like 6 or 6.5 inch, with tweeters mounted high on the front doors. Any speaker will sound better with more amplification. You should be able to drive at least 25

***clean*** watts per channel through each of the stock speakers. That's not much. To do the job right, you'll want to replace the lower speakers. You might do OK leaving the factory tweeters, though.

I emphasize "clean watts" to suggest another thing: You'll need to use separate amps. There is no such thing as an in-dash unit with enough power to properly drive a system.

The next thing I'll say comes from being a bass player: Putting in a subwoofer without first making the door speakers sound awesome is a total waste of money, time and space. The subwoofer will produce extreme lows, but without beefing up the existing speakers & amplification, there'll be an enormous acoustic gap between the frequency where the subwoofer leaves off (at the high end) and where the door speakers pick up (at their low end). This is why most vehicles with subwoofers sound like crap, and why (if you ask musicians who still have functioning ears), subwoofers in cars are a really dumb idea.

Put your money FIRST into a good dash unit with pre-amp outputs (4 of them, one for each speaker), good outboard amplification, and replacement door speakers. I think you'll find that once this is done, you won't need subwoofers. And, your truck won't be such a magnet for thieves.