If you could write off, which most cannot, you multiply the write off by your tax bracket to get your net. It's not like you're getting $500 added to your tax refund.You typically cannot take the charitbable tax deduction unless your itemizing your expenses anyway. Most people do not anymore with the 2017 increased standard deduction.
Best bet is take it to the wrecker yourself if it moves at all or you have a way to move it, or just sell it on craigslist or whatever.
There is some romantic notion that your junk car will be fixed up for some poor downtrodden soul so they can get themselves to dialysis treatment. That doesn't happen. It gets chopped up for the valuable parts and metals. Maybe some of that money gets donated to the advertised charity.