The PTT pin on the 5105 ACC connector is, I believe, an input line. I use it as such when driving my rig with a WOLPHI interface for PSK.
What might work well is an RF sensing TR switch as included in many amp designs. Here's a kit for one:
2" square. One small box with four BNC connectors and a cable to your 5105. It doesn't say how much current it draws from your source of 12v.
For $20 I wouldn't be able to buy the parts.
Mike
