Repair of the seat belt interlock components is limited to their replacement. Testing the system requires the use of a special tester, which connects to the seat belt/starter interlock system at several points. About the only place that you are likely to find the tester is at your local Subaru dealer's. Because of this, repair and testing are best left to a dealer.
If the warning light bulb burns out, access to it may be gained by removing the instrument cluster.