Initiate a paper trail - write a letter to the owner of the shop (it's somewhere in the business registry web site - FL has one) indicating the vehicle needs to be completed in two weeks. Point out your displeasure in the progress and the time it has taken. Indicate you WILL be picking it up, regardless of state, and that a price adjustment will be made for work not completed.
Have a tow truck lined up if needed - cut your losses.
It's been four months. No more showing up and pointing to the issues. Brian's right, you need to document everything and write things down that need to be completed to include;
Interior detailed
Grit removed from paint
Blemishes
Stripes
Decals
cc: Yewl, Bee & Screwed, attorneys at law.
That might get their attention.
I've never sued anybody, however I know that at some point the smiles between people require documentation detailing the sequence of events that otherwise will serve as a record of what's transpired. Legally, he said/she said means nothing. Certified letters documenting whats required per the terms and conditions of a contract are what holds up in court. If this guy has screwed up your car letting his kids and nephews paint their first car, why should you be out of pocket for their incompetence?
I've had many a car scuffed, primed & painted and completed in well under five business. Granted, they were $1000 paint jobs. If I paid $3000, it'd take three to four weeks ...but not four months.