I stress here that this is what I had to obtain. I came across an instruction document from the Indonesian Customs authorities that identifies the requirements, which unfortunately is in Bahasa Indonesia. If you require a copy, send me an email and I'll return the Acrobat version. It may be a useful document to give to your customs broker in Jakarta, letting him see the official Customs department requirements.
Firstly, prepare a little typed story about yourself. Skip the parts about your cats, dogs and your music collection. Move straight onto what you're driving, where you want to travel, how long you'll be in Indonesia, and where you intend to exit the country. You should present this letter to the Ministry of Tourism and to the Police.
You'll also need somebody to translate and to get you to the appropriate offices of the buildings you visit.Your customs broker should be able to offer somebody suitable. You should be with him at all times: don't allow him to apply for this paperwork alone. Your face will open doors. Tourists are generally exposed to an entirely different face of government departments compared with regular expats doing business in the same country.
Ministry of Tourism
You need a letter stating that the Ministry of Tourism is quite happy for you to travel around Indonesia in your own vehicle.
This is obtained from the Ministry of Tourism, which is located SW of the Monas monument compound, on the west side of Medan Merdeka Barat. It's two buildings north of the roundabout, next to/north of the Indosat building.