Let's play "Stump the Experts"

Here's the problem:
I installed a Terk XMCommander on a '82 SD with original Becker head unit. The XM kit has an antenna splitter that interrupts the radio antenna to
allow a FM feed from the XM receiver into the radio. However, when the XM unit is hooked up, I get great XM and FM but no AM--just static. To problem solve, I unplugged the power cord to the XM receiver and AM works. I can unplug the FM out feed from the XM receiver and AM works. Why do I just get static when the XM receiver is integrated into the sound system? Better yet, how the hell can I fix it, Oh Wise Ones?
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