Hi! These are common to a lot of years. My '68 300 had a fully retractable factory antenna, does yours? Likely, the cable broke. They can be repaired commercially; I do believe I repaired my 300F antenna using weed whacker cable. I think the issue was attaching the cable to the antenna, but for the like of me I'm drawing a blank. There isn't much to these otherwise. These pop of on eBay and can be tech-sourced in the Chrysler 300 Club websites; there are 2 different but excellent 300 clubs that can help. Also, Murray Park in Ohio probably has a unit should you go that route. My '64 Imp has the same unit as the '68 Chrysler.
What I CAN tell you is this: if you try to use an aftermarket "compact" type power antenna that comes with its own switch and works automatically once the radio is turned on (or off/retraction) beware, the OEM map light-antenna switch will NOT NOT NOT NOT work. There is a way to convert these aftermarket antennas to work with the dash switch but it requires two relays wired correctly and a bunch of connections, but it will work. Ask me, I know.....and I have the schematic if needed. I wish I could remember how I repaired the F antenna but memories are lost so often and that was pre-cell phone picture taking. Let me know if you need that schematic. Those aftermarket antennas are $40 or less. I eventually went that route when my mast went south on my '68....with the relays in place it works like the original and from the dash switch.
Any body got anything else here? Antiquity is affecting me more than I know these days.....
Welcome to the site! It's grand and informative!!!!!!!!
bob