AT my wits end.

You screwed the mag mount to the cab??

 

Check where your ext. spkr is plugged in at. Right behind the cab is not optimal. About 18" - 24" will work better unless your truck is like mine and has a poor rf ground on the bed. I ended up drilling a hole in the roof of the cab. What the heck, it's a pickup and won't hurt the value at all when you trade it in......as long as you put one of those rubber plugs in the hole so it won't leak!

 

I must be confused mike lol, he said he first used a fiberglass whip behind the cab, which is not optimal as you said. Then he changed to a mag mount, but never said he switched back. Does sound like ground issues though.

 

So..with the mag antenna hooked up, you turn the radio on, have recieve, and weather, then it slowly fades out after a few seconds? Turn the radio off, then back on, and the same thing? What about transmit, does it work in this pattern also? Or can you tell?

 

Could be a couple different things. A short in the coax or in the mag mount. A short in the mic. Could be a bad component or cold solder joint in the radio. Its just hard to say from here. Are your antennas and coax new, or known to be good?
If so, first thing I would do, try a different mic.

Second...You could try moving your tram antenna and mount towards the rear of the bed and see if there is any improvement. Antennas don't like any vertical metal any where near them, that's why mounting directly behind the cab is not the best place to mount one. Some guys do, and can achieve decent swr, but performance suffers greatly compared to other mounting locations. You want your antenna up as high, and as far away from any obstructions, as you can possibly get it.

 

He might be better off just taking the whole setup to a good CB shop and have it checked out.