Temp gauge not working Chevy C15

[TCLARK]

No evidence of holes anywhere, not even welded over,I
think I will plumb the tube down the center as suggested.
I should visit the sister truck about 45 minutes east, and have a look;
I would also like to see if it has capillary tube or electric
heat gauge. The truck is identical to mine but is for parts only.
The owner wants me to buy it but there is nothing I really need
on it other than an inside door handle!

I will post a pic of a Ford
wiper plumbing, taken at an old farm junkyard in S.Alta.
Cheers
Terry