300E 4matic - Returning to Function Mode

[MeAndMy190]

My brother's '92 300E 4MATIC system was placed on test mode for about 2-3 months, on the advice of our Mercedes tech (trying to prolong the system's usefulness). The system worked fine before putting it in test mode with the exception of some minor vibration when running at low speeds with the wheel sharply turned. Upon switching back to "Funktion," the rectangular "4MATIC" light on the dash stays lit and the system remains disabled. The car seems to be driving fine on the rear wheels. (Of course, he waited until we got a foot of snow before trying to reactivate the system). We've tried removing the positive battery cable for half an hour, 2-3 times, with no success. Does this mean that a problem developed with the 4MATIC while it was running on "test" mode, or does someone know another trick that might get the thing activated? We're in the middle of some midwestern winter weather, so any advice would be appreciated.