Just bought used 2003 Lexus SC430 with after market radio, and sound only comes out of the front tweeters. Where should i be looking to fix this?

Loose wire connection. Or take it to an audio shop.

I bought a used Lexus SC430 this past year, and love the car. The only fix that I didn't notice prior to purchase was the horrible condition of the car main speakers in the doors (lower big speakers near bottom of car doors. The car had the Mark Levinson radio, and I missed this flaw, but it became obvious that they were either broken from age or design flaws that did not stand the test of time. I went to a local shop that installed after market radios, and for about $250 replaced them both with more up to date speakers, and all is now well. I feel like the speaker quality was compromised when the car was built. You might just let a similar shop take a look, as the price at the dealer was 5 times the price of replacement for something of better quality. I hope this helps, and it took care of my issue.