THALOUS Bicycle Gear Shift Lever Left 3 Speed Thumb Shifter with Gear Indicator for MTB Moutain Road Bike Tricycle

I am still dealing with my 40 yr old fat tire bike. I can't get a cable started into the left thumb shifter. I ordered a new one but couldn't make it work. Eventually broke it while taking it apart. So I ordered another new one. I can't get the cable started. I keep poking at it but nothing catches. Probably will try attaching the new cable to the end of the old cable and see whether it will feed in.

Any suggestions?

TIA,
kevino