At this point, I have paid about 11% sales tax on my vehicle, purchased originally in 2019. I originally paid 7% in Indiana and another 4% when I registered my vehicle in New York. I want to know how, and with which taxing body, to redeem the credit I have been given to get me back down to New York's 8% sales tax.
You don't.
If you were a resident of Indiana when you bought the car, you properly paid Indiana sales tax when your purchased the vehicle. When you moved to NYS and registered the vehicle, you should have included a form DTF-803 Claim for Sales and Use Tax Exemption with your registration application. You should not have paid any NY sales/use tax.
If you were a resident of New York
when you bought the car in Indiana, you would file form DTF-804 instead. You get a state x state credit of 4%. In other words, the state of Indiana charges 7% and NYS charges 4%, so the biggest credit you can get from NYS is 4%. You don't get a credit for the county tax (usually 3-4%) because the New York county that charges sales tax does not have its own agreement with the state of Indiana.
Since you are asking about this in 2023, I suspect you were a resident of Indiana in 2019, and just recently moved to New York, and I think you may have used the wrong form to register your car when you moved to New York (did you use form DTF-804 instead of DTF-803?). I don't know if you can fix that, you have to ask DMV.
If you were a resident of New York in 2019 when you bought the car in Indiana, and are only getting around to asking about the sales/use tax issue 4 years later, you don't get an adjustment for the county tax. Indiana state charges 7%, New York State charges 4%, so the most you can get back from New York State is 4%. The New York county sales tax would have to come back from the county, but I don't think the counties do that. You would have to ask your county.