Believe it or not, the mysterious blob in the photo above was serving as a firewall grommet for the speedo and tacho cables. Someone must have thought this was a good idea when they did it, maybe they were planning to order the correct grommet and just forgot?
There’s a lot of discussion online about which grommet is correct. I ordered the one on the left from Corvette Central and it was a perfect fit. The trick is to make two cuts (from each hole to the edge of the grommet) so you can just slip it over the cables. I fitted it from the engine compartment (not from under the dash) — it only took a few minutes. The alternative is to undo the cables behind the instrument cluster and then use special tool to push the cable ends through the grommet — a lot of trouble for very little benefit.
The hardest part was cleaning off the residue of that sticky rubber blob that was all over both cables.