Bill - this is one of many times that you really should have in your possession the Goldwing Service Manual. It will help for situations just like this and much much more.
You will need to remove all the screws the hold up the upper trunk liner to access some of the screws that hold the speaker pods in place. You will also note that there are several screws that are on the underside of the speaker pods themselves too. While not an extremely difficult task, it does take a little time, and you need to be very careful to not loose any of the screws down in the bike itself. Also pay particular attention to which screws came from which places as there will be different sizes involved too.
Go buy the manual though and use that as a guide and you will be much much happier with all your maintenance and parts removal tasks.
Bill, Dale has some good advice. Actually, you can't get inside the rear speaker enclosures (ie, between the mount and the grill), unlike the front speakers. The rear grills are not removable. If you're mounting a speaker, the screws pass through the speaker flange and into the nuts already present (if I remember correctly). The speaker "hangs" from the mount. Not a good description, but the best I can come up with right now.
Not if you want to secure it by reaching it from the inside. It's just a rather involved process that involves lots of screws and bolts, so it's a nice job to have behind you. Other tasks are more "fun".