I'm guessing you want to mount the rear speakers of the convertible into the same spot but into the coupe?
If your doing this, yes it is possible but you'll need to take the seats out, take off the plastic that the speakers are attached to and do the same on your coupe but hack open new holes for the speaker panels. The speakers are 6 1/2's and the ones on your rear deck are 6x8's. Doing this is quite the hassle to have extra speakers, but it can be done. If you are sure of what your doing my advice is get some new 6x8's for the rear deck or 6x9's and customize the hole.
For mounting the amplifier this all depends on if you have subs/ if you are in need of lots of trunk space. Personally I would mount them on the back of the sub box but that's me, you may not even have subs...
Please explain more of what you are doing and I can help you further.