This guy is selling NOS temperature gauges, but note that he's in Egypt.
Shipping took a while but I finally got the one I ordered.

TheSamba.com :: VW Classifieds - VW T3 Vanagon Temperature Gauge 251919511A
NOS <https://www.thesamba.com/vw/classifieds/detail.php?id=2362467>

On Fri, Aug 6, 2021 at 12:38 PM Dennis Haynes <d23haynes57@hotmail.com>
wrote:

> If the gauge is reading normally with the low coolant controller (43)
> removed than the gauge is the problem.
>
> They are all similar in age. I forgot the value, try replacing that
> capacitor in it.
>
> Dennis
>
>
> -----Original Message-----
> From: Vanagon Mailing List <vanagon@gerry.vanagon.com> On Behalf Of
> OlRivrRat
> Sent: Friday, August 6, 2021 11:14 AM
> To: vanagon@GERRY.VANAGON.COM
> Subject: Re: Low Coolant Warning LED
>
> Coolant Level Full @ all conditions
>
> Wires to Snsr Disconnected & Jumped > Flashes
>
> Wires from Snsr Ground Continuity > Good
>
> Wires from Snsr to Relay 43 Continuity > Good
>
> Wire from Relay 43 to Ins’ Cluster 14Pin Connector Continuity > Good
>
> 3 Traces to Gauge on PC Foil Continuity > Good
>
> Swapped In ( now ?able ) used Gauge > Still Flashes
>
>        Guessing @ this point that must have 2 Gauges with Same Symptoms &
>
> Quite Actually Not Surprised @All if that is the case ~
>
>         1 additional note > With Relay 43 Removed > Still Flashes
>
>
> > On 6 Aug , 2021, at 6:57 AM, Dennis Haynes <d23haynes57@hotmail.com>
> wrote:
> >
> > The initial key on indicator test is part of the circuit in the gauge
> itself. There is a capacitor that controls the timing of this feature that
> tends to fail. The other possibility is that the low coolant sensor control
> is signaling the low coolant light. There are 2 versions of this
> controller. The early ones would short the temp sensor to ground causing
> the gauge to rise and the light to flash. The later units' "pulse" the
> sensor ground causing the light to operate with the gauge rising a minimal
> amount. This change allows an indication for coolant loss vs. over
> temperature.
> >
> > It is also possible that this could be indicating a true cooling system
> issue. If the system is low on coolant or air-combustion gas bound on a
> cold start the coolant will be pulled down from the tank causing the low
> coolant indication. Turning the engine off, especially with a non
> functioning pressure cap will allow the coolant to flow back so when
> checking the tank it appears full. During this light on condition leave the
> engine running while you check the coolant level in the pressure tank.
> >
> > Dennis
> >
> >
> >
> > -----Original Message-----
> > From: Vanagon Mailing List <vanagon@gerry.vanagon.com> On Behalf Of
> OlRivrRat
> > Sent: Friday, August 6, 2021 12:39 AM
> > To: vanagon@GERRY.VANAGON.COM
> > Subject: Low Coolant Warning LED
> >
> >       Could someone please explain what controls the initial Igni’ On
> Low Coolant Warning LED
> >
> > Flash Duration ~ Have 1 that won’t stop blinking,, Have performed all
> the normal diag’s including
> >
> > swapping in diff’ gauge ~ Am wondering if it might possibly be that what
> ever turns off that initial
> >
> > Igni’ On Flash might be malfunctioning ~
> >
> >
>  ORR ~ DeanB
>