The speakers on my daughters 1998 ES330 no longer work but the tweeters do. Best Buy replaced the stereo 4 years ago and everything worked fine. I'm thinking maybe it's a bad amp. I bought a used one off eBay & got intimidated by all the wires. I haven't started the replacement process yet. Any advice would be appreciated.

 

I had the same issue on my 99. The wiring should not be too bad, if I remember right there are 2 nuts holding the mount for the amp, then a few nuts holding the amp onto the mount. There are 2 or 3 plugs which should be easy to undo. You may need to pull the glove box, which is not too hard.
I bought 2 new amps from my local scrap yard, none of them fixed the issue which leads me to believe it may have been the radio. Instead of fighting the issue, I went aftermarket. Now I have a Kenwood in the dash, and everything is working great.

 

I would first take a multimeter put on 'AC volts' (low scale) across the speakers' terminals, turn on the head unit(radio) turn volume up 1/4 +, and look for a reading. If you get a reading, then it's the speakers that are bad. You could also then disconnect(radio off) the speaker wires from the speakers in question, and then put the multimeter across the 2 speaker terminals with the MM on 'Ohms' (low scale) to see if the speakers' voice coil is "blown" (bad). Gotta have a multimeter in your toolbox! It'll save you $$!