Not real happy with autometer

Sorry Autometer didn't warn you

Something you might try, what is the speedo, two insulated conductors from the sender?

If you cannot get shielded wire, pirate some RG-59/59 coax from someone, strip off the insulation, and gently compress the shield while still on the inner cable. This will expand the shield and you can thread your sender wire through. You'll have to re-insulate it even if you have to tape it

Another alternative is a "chunk" of ethernet cable. These are 4 pair, blue, orange, green, brown. the mating wire is white/ blue, white / orange, etc, and these whites are twisted around their mates.

Take all the /whites and tie them toegther and ground to the speedo case. Use the solid blue and solid orange for your sender, and also ground the end of the remaining two, ONLY AT ONE END, at the speedo.

Some of these gauges are a "bridge" circuit, and the gauges MUST have a clean ground, something that stocker Mopars do not need.

You might try getting a little filter network for the 12V going to the gauges. Try a two-way shop ask them if they have an "interferance filter" for "in series" with the 12V line. Usually these are an inductor choke in series, and a capacitor across the line to ground. If you can't find, dig up a couple of ferrite core filters. If you have any old computer gear around, you should find one or two. Monitors always had them in the video cables, that big "bump" in the cable Pass your 12V line to the gauges through there, take a couple three turns through the core if you can.

Go to Rad Shack and get a couple of .01 disc ceramic capacitors. Install them from the 12V feed to ground, IE across the gauge.

We used to see this a lot in Diesel over the road trucks with pyrometers, which are a low current, low voltage, "self generated" circuit using a thermocouple, and VERY prone to static/ noise pickup from the truck's electrical system.