12V trigger wire

Hello, this is my first time posting to the forum so please excuse that I am a newbie. I decided it was time to upgrade my system and start listening to more music again.

So I have a Parasound P5 preamp and an A21 Amp. The amp is across the room close to the speakers, so I have two 50ft balance and shielded XLR MIC cables connecting to the preamp.

I want the amp to turn on when I turn on the preamp. So I got a 50ft mono wire with the 3.5mm jacks I need.

The P5 has a 12V trigger wire output, and the amp has the trigger wire input. I connected the two with the wire running from one to the other in the room, and it worked fine. Turn on the preamp, and the amp turns on.

But after I ran the trigger wire through the attic and down the wall, it no longer would work. I am thinking that perhaps the trigger wire is picking up too much electrical interference from the MIC cables along the route, or other wires in the attic. Is that likely the problem? Is there a certain type of trigger wire I should look for that is shielded better, or will be more likely to work?

Thanks!