I've tried to fit a Pure Highway 600 system through FM on my concert radio.  I can get the radio to store the recommended 87.6 frequency, but it lasts a very short time before it starts a continuous repetitive audible beep.  Any ideas?

Oo you have me stumped. To be fair I get the beep occasionally but never understood what it meant. It mainly happened to me when the engine was off but radio on so I assumed it meant the battery was low. 

I've worked it out and it makes me feel sooooo stupid.  It was the TP button, the radio was searching for frequencies with up to date traffic information, that's what the beep was. I held down the TP button and my Highway automatically connected. That's a Sunday I'll never get back

Glad you got sorted ☺️