I am out of ideas, other than Google around your meter / model and look for other user's reviews/ problems, etc
You have any access to coax, TV, or CB? Not critical. Cut two pieces each one about a "handy" length for your measurements. Buy some Banana plugs on Egag to fit your meter. Tape the two coax together (or zip tie) and tie the shields together at both ends
At the meter end, with the shield of the two pieces connected, tape that shield off. Strip out some of the center conductor and hook the banana plugs up to connect to the meter
At the engine end, you want a short ground clip on the shield. This will connect near the dist. to engine ground. Hook the appropriate center coax lead to the dist. tach connection, and the remaining coax center lead to whatever the meter wants......12V, ground, whatever it should be
This creates what is known as a "Faraday shield"
============================================
You could also do this with a scrap ethernet cable.......they are 8 conductor IE 4 pair. Select one pair and use them for the meter connections.
Take all other wires at tape them off at the meter end, same as the coax, above
At the engine end, twist all unused wires together, put a clip on them and connect to engine ground. That is your shield
