How to replace the Becker Europa radio without screwing up the Hershmann antenna

I'm planning on replacing the Becker Europa radio in my 81'240D. The replacement will be either a Sony or Kenwood (something without the bling). However, in discussing the installation with several places (Best Buy, etc.) the majority seem confused as to how to hook up my fully functioning antenna without screwing it up. Since many of the install people seem to be on a different planet, where can I find some info or is there a place to go that is knowledgeable about such an install? Any input is appreciated. Mike