Sounds like the OP has been chosen as the arbitrator on behalf of the vehicle owner, he and the MPI appointed arbitrator were not able to come to terms, so the next step is selecting an umpire who's say is binding on both parties. That said, he's looking for suggestions on umpires suitable to handle the situation.
That being said, good luck finding someone both qualified and unbiased that the MPI arbitrator will agree to use. Maybe the sales manager at one of the local Subaru dealers would take it on? It does pay afterall.