The road to the observatories on top of Mauna Kea is steep and gravel. It is no joke to drive. The visitor center posts photos of vehicles that missed the switchback turns and went off the road.
The rental agencies on the island do not want you to risk their cars on that road, and in fact, don't want you to drive the road between Kea and Loa. They make you sign a release. So if you feel confident and you take the liability, you'll drive the rental cars to the top. Other than Harper's 4WD, I don't know of any other services. Ask around in town. You should be able to find a local to do that for a fee.
Once parked on top, you still need to stroll over to the highpoint hill, which has caused several people to choke in the thin air, being at 13,000' and all.