I will be changing the head unit. I was also thinking about buying 4" for the dash. I was wondering if you guys know if the change in head unit will disconnect the dash speakers.

IE:
I had a 90 celica with a 12 speaker system (factory) and when I took the factory head unit and replaced it with an alpine the 8" woofers on the doors stopped working.

 

Different vehicles are wired differently, on your celica, I am going to assume that you had the infinity system and on those systems if you change the radio you would have to wire up the factory amps that power those door woofers. Older Bose systems worked very similar. If you are working with the Scion system then it may depend on if you have one of "premium" systems either pioneer or alpine... Up to the 2010 scions most should not need any kind of wiring bypass. One exception to that may be the new body style TC with the three way door speakers which more than likely will have an amp for the woofers or a crossover that will need to rewired. Hope this helps a little, good luck and keep us up to date on your project.

 

I actually have a 1st gen xB.
Thanks for the insightful info. It helps a lot. I'm going to hold off on purchasing those 4" Speakers until I install the head unit. I'm pretty new to stereo items and I figure this car would be a good car to learn from.

 

I've replaces all the speakers on the xB and they work...
don't see an issue.

 

Even the dash speakers? I'm not worried about the door and rear speakers.