If the TIU channel is shorting, the top suspect on fixed channels is the TVS protection diode. They're a large thru-hole component and fairly easy to replace. It's not uncommon for those to fail shorted.
Here's a picture of the TVS diode locations in the Rev. G, they're in similar locations on the later versions, but I don't have one apart to check. Pretty easy to spot, they're large cylindrical components.