My Mercedes runs and then just won't start , changed crankshaft sensor and ran for a week....but started up again before I changed it and then a week later wouldn't start again..was told fuel pump so changed that n fuel filter ran fine for 2 weeks, then went to store ,came out n just turned over would not start and still won't, mechanic came over wired direct said fuel pump was fine, started right up when he direct wire from battery, said all my fuses and stuff were good but power coming from underneath fuse was not working when putting test tool on it, the fuse to the plump is not blown nor is relay , what could be making it not get power in the wiring ...please someone help!!