IVR- What does it look like?

IVR in some of the early cars is inside the fuel gauge. Tipoff is fuel gauge has 3 terminals. One is 12V feeding the IVR, fuel gauge is internally connected to the IVR output, and the IVR output feeds to a stud to jumper to the other gauge(s). 3rd terminal of course is sender. There are many internet posts on various ways to replace. "Basic" is to safely lock off the internal one such as RTV to hold the contacts open, etc. Then feed an external IVR to the proper gauge terminal which goes to the other gauge(s)

Two of many. Do a Google

RTE Gauge Faq - rte

Gauge Cluster Issues/IVR

FYI the original limiters worked sort of like a turn flasher only more delicate. They constantly cycle on/ off to form pulses of power that roughly cut the 12V in half.