Issue with Antenna Light... Transmit and Receive

I just was testing out a co-workers CB in my truck because he is having issues with the antenna light and I was thinking maybe he was just having antenna issues and I know my antenna and CB combination work no problem... The problem with the radio is that when he is stationary and transmitting he has no antenna light going off and transmits loud and clear... He was transmitting and receiving for a long distance when he was unloading and I was going back to the City... I transmitted my radio (with amp and peak/tune) all the way to 16miles with him still hearing me with some static... I can't remember how long his transmitted for but he went probably 8-10 miles (no peak/tune or amp)... So anyhow forward to the following day we pass each other on the highway and all I got from his radio that time was "Hey Trucker" and then he cut out.. We weren't even 1 mile away.. The change was he was moving... So anyhow his radio does the same thing in his truck as it did when I tested it in my truck..

I am not to sure what is wrong with it... It doesn't make sense because it's not like the CB can sense a moving truck or not.. I just can't see why it works perfect when not moving but as soon as truck gets moving it screws everything up....

Any ideas?

It's a Cobra 29LTD Classic only upgrades done by previous owner.. upgraded to blue lights (from green) and echo board other then that it's stock...

Thanks