I'm trying to refinance my vehicle. Is it true that a co-borrower will not help my credit in the long run?

I've heard that adding a co-signer only builds credit for the co-signer. Is this true? I'd like to refinace because I'm paying an astronomical rate, but no one will give me a new loan yet since my credit is still building back from bankruptcy. I've had the vehicle almost 6 months and I got the loan on my own. The finance manager told me to try after i've had it for 6 months, but I haven't seen an improvement in my credit score yet.