I agree with 2 Many;
You should either get YOUR radio,done as YOU asked and they agreed to,or you should get YOUR radio and ALL your money back. Do you still have any of the emails you sent archived? If so,you should print 'em or copy 'em to a disc and save 'em somewhere safe;you might need them to disprove their claim that "you never asked for a 12 volt conversion." I think you've given them every chance to make this right,and they're blowing it off. (I'll bet they owe you about a pint of that expensive radio paint,too.I can't imagine they'd use a whole quart to paint one radio case.)
If it happened to me,I'd send a registered letter and tell them I'm not happy at all with their work or their business ethics,and I'm offering them ten days from the post date to either do the job RIGHT,or send my radio and remaining paint,properly sealed,back AND credit my account for all money I paid. I'd remind them that I contacted them by TWO different means to discuss what I wanted done,so it's not like they weren't told. I'd say I have my communications with them documented so if they want to fight it I'll have my attorney contact their attorney,and then it'll REALLY get costly for them to settle. Maybe I'd indicate I'd already made an inquiry to the BBB about them,so I guess I could call them AGAIN,let them know what transpired from this letter. I'd send them a COPY of the letter,also give one to my attorney (or save one for an attorney to be named later),and keep the ORIGINAL for my files.
Just be thorough,polite,and direct.Don't make threats,just tell what you're prepared to do if they don't comply. Man,I hate businesses that pull this s**t on good people! If I could go back in time,I'd be an attorney,and I'd represent people who get things like THIS done to 'em!
Speed