The source writes:

As of 1 July 2008, motor vehicle tax revenue is attributable to central government, so a new times series BU2004 has been added to our database.

In response, we have archived the predecessor (January 1991 to September 2009) by marking the metadata and catalog entries as "discontinued." The replacement starts at July 2009. Figures are millions of euros (Mil. EUR) not seasonally adjusted (NSA).

The series reside in the historical catalog (Germany » Government » Central, State, and European Union Tax Revenue by Type) and are:

• DBBBU2026M.IDEU = [DISCONTINUED] Monthly Report: Public Finances in Germany - Central State and Local Government Individual Taxes; State Government Taxes; Motor Vehicle Tax, (Mil. DM, NSA)
• DBBBU2004M.IDEU = Public finance: Motor vehicle tax, (Mil. EUR, NSA)

Because catalog locations are subject to change, the upper-right search box on DataBuffet.com provides a "find in catalog" mode that accepts a mnemonic.