Rear speaker install options

[mva1]

I am wanting to install rear speakers in my newly acquired Polaris Ranger 1500 Northstar. I know there is a set of wires ready to go under the headliner and I have a set of mounts from UTV stereo. Once installed if i plug them in is that all I need to do and if not what else do I need to do. UTV Stereo is the worst as they don't answer their phones and don't respond with real answers to questions e-mailed to them. Can I purchase any quality set of marine type 6.5" speakers? Any response and recommendation is very appreciated for this newbie.