Received CP2000 due to unreported cancellation of debt for car auto loan. As a co-signer, can this be disputed?

yes

the co-signer only has an obligation to pay the loan if the primary borrower defaults.  The IRS recognizes this, and clearly excludes guarantors from the cancelled debt reporting requirements in Treasury Regulation 1.6050P-1(d)(ii)(7):

(7) Guarantors and sureties. Solely for purposes of the reporting requirements of this section, a guarantor is not a debtor. Thus, in the case of guaranteed indebtedness, reporting under this section is not required with respect to a guarantor, whether or not there has been a default and demand for payment made upon the guarantor.

therefore you need to respond to the notice saying that under the above reg (cite the number) you are not required to report this as income since you were the guarantor and not the debtor. This is also established in case law such as Landreth v. Commissioner, 50 T.C. 803(1968) the U.S. Tax Court.