does anyone know if a 6.5" round speaker will fit in the 6x9" rear slot of the magnum. maybe with a custom bracket or something. is there enough room?

 

I think it depends on the speaker.

I removed the 6x9"s in the front on mine, and a friend made some custom mounting brackets from aluminium for me to mount 6.5" speakers in there. Worked fine. For my polk 6.5" speakers, I know they would fit in the back using the same brackets.

-Dave