Are you trying to replace a broken antenna? If so, you don't need to get in there. You can just replace the mast. Remove the nut on top of the antenna, pull the old mast out. The new mast has a nylon track that will feed into the antenna base. It is a 2 person job, but if you have someone turn the radio on, so the mast would have been extended. Then you insert the nylon track into the antenna hole and have the person inside turn the radio off. It will pull the antenna in. Put the nut back on the antenna and you're all set. The mast will come with instructions too. Look on
ebay for the mast, you should fine them for a few bucks. I've replaced a couple of mine already. It's a whole lot cheaper than a new antenna.