Maybe needs more power.
There has to be enough current to heat the resistance wire.
My sequence for testing the gages alone when the gage or instrument cluster is removed.
First check each gage for resistance. That will confirm the wire is intact.
I show that here on a 67 Rally panel.
https://www.forabodiesonly.com/mopar/threads/gauge-cluster-issues-ivr.455893/#post-1972855388Then I checked each similiar to what you just did but using D-cells.
Finally I check the system semi-live using a car battery with breaker to power the IVR, and 22 ohm resistors in various configurations to represent the three test points for each gage, 10, 23, & 74 Ohms
See this post and the the last one in the thread.
https://www.forabodiesonly.com/mopar/threads/gauge-cluster-issues-ivr.455893/page-2#post-1972858065Here's the 1966 Master Tech on Gages
http://www.web.imperialclub.info/Repair/Lit/Master/227/Page01.htmand the filmstrip
https://youtu.be/mpkrm3QKCjcThere may be an earlier MTSC on gages - ypou can skim through the Imperial Club Collection, mymopars, but the concept will be the same as covered in 1966