If you go down in the tunnel (sorry but I can't remember whats it's called >.<) in Goldenrod, you will see a person who will have Team Rocket costumes that you can put on. Go to them and put a costume on, then go back to the radio tower and talk to the Rocket guy standing in front of the door. He will think that you're a member of Team Rocket and start to let you in. But then your rival will come and ruin everything (LOL that got on my nerves) and you get inside after a bit of chatting and fighting!
If I completely understand your question then this information Should guide you easily.
Source: Experience
