Alpine cdm 7874 issues

[k1500 97]

(97 silverado) Ok so I have a Alpine CDM 7874, the problem I'm having is when I have the FM radio on it works fine through all speakers besides my pass rear which is blown, I have a m bus blue tooth receiver installed, so when I turn on the blue tooth only 1 speaker works n I have to smack the dash or move the unit to get all of them to work. I've replaced the plug 3 times, same issue, any help is appreciated and any product recommendations is always appreciated. Thank you