I had a 71 with the fender mount. I don't quite recall how I disconnected it but for sure the antenna is held to the fender with a thread on lock ring. I'm pretty sure I held pressure on it by pulling up on it while unscrewing the antenna, lefty loosey. On the inside just unplug it and pass it through. Once again, don't remember if the cord unplugs at the antenna end, think its hardwired and just feeds through from the inside. Hope that helps, good luck.
I think you might be able to reach the lockring through the firewall where the electrical passes through, that might be how I undid it.