Exhibit 10.13

ACKNOWLEDGMENT OF GUARANTOR

BSST LLC hereby acknowledges that (a) it previously entered into a Guaranty
dated October 28, 2005 in favor of Bank with respect to the obligations of
Company and (b) Company and Bank have executed an Amendment dated as of date
hereof (the “Amendment”) to such Credit Agreement (the Credit Agreement as
amended thereby, the “Amended Credit Agreement”). BSST LLC hereby ratifies and
confirms its obligations under the Amended Credit Agreement and the Guaranty,
and agrees that the Guaranty remains in full force and effect after giving
effect to the effectiveness of the Amendment, that BSST LLC’s obligations
thereunder are not subject to any defense, offset or counterclaim and that, upon
such effectiveness, all references in such Amended Credit Agreement and the
Guaranty to the “Credit Agreement” shall be references to the Amended Credit
Agreement. Capitalized terms not otherwise defined herein will have the meanings
given in the Amended Credit Agreement. This acknowledgment shall be governed by
and construed in accordance with the laws of, and be enforceable in, the State
of Michigan.

 

BSST LLC By:   /s/ Sandy Grouf Its:   Chief Financial Officer Dated: April 29,
2008