XM Radio no signal

[djbmsu]

2014 Phaeton with JVC Radio, everything works perfectly but the XM will come and go and show "no signal" then come back and play perfectly. Underneath the dash I found two black box's one larger one smaller with everything connected except one cable (see picture) one cable goes to the radio, could the other that is NOT connected go to the roof antanae and I cannot find a cable to connect to it ? any comments ?

[94-Newmar]

Don, what you have pictured is the protocol adapter. This allows aftermarket manufacturers to use the XM. The antenna is not connected directly to this unit. The wire not being used on the left is to connect to another "head" unit. See the small wire on the right in your picture? That wire connects this protocol adapter to the XM tuner box (smaller black box) On the tuner box is the plug-in for the XM antenna.