My brother-in-law replaced his rear speakers with 3 way speakers, and there is no high-end coming out. They seemed to be just a mid-bass, but I can't find the crossover. The front speakers have the amp with the x-over right by the speaker, but I can't find anything on the rear speakers like that. I have looked in the rear by the spare tire and couldn't find anything.
Does anyone know where I can find the x-over for the rear speakers?

 

Does he have a stock radio, or an aftermarket radio? Those speakers are for mid bass only (as you've figured out) in stock form. These locations are mainly for back fill and your soundstage should be in the front. I say move the speakers to the doors if he hasn't already replaced them.

 

He has an aftermarket one. I did find another post about where the amp was. For anyone that's wondering, it's on the passenger side underneath the rear seat armrest. I just cut the wires where they went into the amp and directly connected them to the speakers. I capped everything off and then plugged the amp back in so that nothing else gets affected. Surprisingly, the bass sounds better than it did! The levels are the same, but I also get mids and highs as well. It sounds a ton better.

 

Glad you figured it out. I just ran all new wires, bypassing all the factory garbage