Ewwww...I'm starting to feel a little guilty of aiding and abetting, Tom. Good improvisation on the gear tab -- I've done some similar kludges on both my earlier Hirschmanns. Maybe you can still cancel the order for the the replacement gear!
Does the antenna shut off like someone threw a switch -- that is, no struggling and straining, it just stops? It could be that the switching mechanism is out of kilter. On the 6000u the switches look like a couple sets of distributor breaker points and are mechanically actuated by a small idler wheel. I had trouble with one set of switches that had become badly pitted (fixed 'em with a jeweler's file). That may or may not apply in your situation -- I confess I'm not up on "modern" cars like yours, and the Antennaworld pdf offers little detail on the switching mechanism.
If I may offer a suggestion, I'd study the mechanism and figure out what makes it start and stop. When the head unit sends 12 volts to make the antenna extend, how does that direct the motor to start pushing the antenna mast driveshaft? Then, what makes it stop -- a cam? some kind of torque sensor? Run it a bit with the cover off and watch it work. Watch your fingers if you poke around the workings with the antenna energized!
I've got an '87 Audi with a Fuba power antenna -- the design is brilliant, but prone to wear. I think I spent about $125 on parts to get that one running right. The Hirschmanns are repaired far more economically, I think (hope). ;-)
Cheers Russ