I posted this in another thread, but it's easier to just do it again.
We have the Axxera that had another set of outputs that we could tap into. A SUPER audio shop made a box with two 4X6" oval speakers and covered it in the same type of vinyl that's on the dash. Couldn't be happier with it.
The little black rectangle above the radio is for our XM radio (The "thing" to the left of that is our tire monitor). The same audio shop did that install also. We take the radio out & use it in one of our cars when we're not in the motorhome.