My 86 Wolfsburg Weekender MV has a clock instead of the tachometer the
GL trim comes with. Now, I could probably buy an instrument cluster
someplace and replace the whole set, and that would look nice. But I'd
like to keep the speedometer stock, and I'm not sure the wiring
harness would work the same for a new cluster. Does anyone have
experience adding an aftermarket tach to this or any vehicle?

I am somewhat tempted by the a-pillar clusters that the honda tuner kids have on
their rockets....

This looks like it might work:
http://www.egauges.com/vdo_mult.asp?Type=Tachometer&Series=Series1&Cart=
when used with this mounting cup
http://www.egauges.com/vdo_acce.asp?Subgroup=Campro_Cup&Cart=