Well here's an update with a bit more information.
While I have not had the time to see if the rear accessory PTT input mirrors the state of Mic PTT, I was able to determine that muting from the XPR4000 radio was not needed as I originally thought. I suspect that the only way to get a TX indication would be to build a small mic extension circuit to allow interception/monitoring of the mic signals.
The problem that lead to this request became apparent in multiple school bus installations using XPR4350 VHF mobiles.
When the radio transmitted, the AM/FM/CD bus radio with Public Address function would "sputter" as long as the XPR radio was transmitting in digital mode. The problem did not exhibit itself when the XPR radio transmitted in analog mode. After some digging it was determined that all of the AM/FM/CD/PA bus radios exhibiting the problem were provided by the same supplier. The problem WAS NOT RF getting into the wiring, antenna, etc.
The AM/FM/CD/PA bus radios were Panasonic AM/FM/CD radios modified to add PA function by Mito Corp and sold as part of the school bus package. They used an amplified microphone but needed an additional boost to the audio before it would properly drive the amplifier in PA mode. This extra boost was provided by a small circuit board containing a LM386 audio amp. The LM386 power was provided from constant 12v and had a gain of about 100. When the radio was in analog mode a very slight decrease was noted on the 12 v line as the XPR radio went into transmit mode followed by a corresponding increase as the XPR radio stopped transmitting. In digital mode this change on the 12v line occurs twice a second as the XPR radio switches on and off producing bursts of RF power on the selected time slot. The LM386 amplifier chip responded to this slight change in the Vcc voltage by producing an audible "sputtering" sound.
The fix was to add a transistor switch controlled by the PA microphone's PTT lead to turn off the Vcc voltage to the LM386 whenever the PA function was not in use.
The added circuit board has gone through several variations depending on the age of the equipment. The first versions I have seen place the LM386 circuit in a small plastic box on the PA mic cabling. Later versions place the circuit board inside a heat shrink sleeve and secure it to the rear of the radio with a tywrap. The current version appears to move the entire mess inside the radio, placing the LM386 on the main circuit board. Fixing the earlier versions is rather easy. Still working on a fix for the current version product.
Considering that DMR and P25 Phase 2 both use TDMA I suspect that this sort of behavior could show up in other installations as well with varying results.
