your stuff must be on the backburner. i would go down there and ask them nicely what the deal is. if he is your friend, take him out for a brew , and find out what is taking so long
This is exactly what i would do.
Don't be afraid to pull the car out of there and find some one else to finish it.
Pay him for what he's done so far, and tell him no hard feelings. Actually, you should compliment him on how happy you are that he's so busy! I had to do this once and it worked out well for me. I finished the car myself.
It's tough dealing with friends though.......
Good luck.