The tires are not "prebalanced". The red dot on the tire is the high point. This should be placed on a radial line with the low point mark on the wheel. If there is no red dot, the yellow mark should be mounted in a radial line with the valve stem hole.

Mercedes (and many other OEM) wheels have a faint grey circle about a quarter inch in diameter on the outside bead flange. Mounting the red dot on the tire on a radial line with the grey "dot" on the wheel will result in the least radial runout of the assembly.

AFAIK this system was first used on Corvette, circa 1970.

Many "incurable wheel balance" problems are due to excess runout and the symptoms are the same as static unbalance - steering wheel shake.

In such cases always check if there are matchmarks as above and whether or not they line up. If the matchmarks don't line up, have the tire reclocked to line them up, then rebalanced. If the matchmarks line up or there are none, have the assemblies tested on a Hunter 9700 system for radial force variation. More than about 15 pounds may be noticeable and certainly 30 pounds is noticeable.

The Hunter 9700 measures wheel and tire lateral and radial runout along with tire runnout expressed as radial force variation and can determine the optimum clocking of the tire relative to the wheel or whether an acceptable clocking to keep radial force variation within limits is possible.

Always discuss "match mark mounting" with your tire tech, and it's also wise to find the wheel matchmarks and mark them with a crayon so the tech doesn't have to search. They are faint and can best be found after thoroughly washing the wheel.

If your OE wheels have been chomed or refinished the low point mark is probably no longer there.

Given your 45 years in the tire businees I'm surprised you couldn't offer a proper and complete explanation of match mark mounting.

Duke