You may have opened the fuse on the GP relay by shorting the leads. Check for supply voltage at each plug. If this checks out test each plug for resistance. If any of them are open you should replace all the plugs and I think that should get your car started!
NOTE:
During the resistance check remove harness from each GP if equiped with series type system. If equiped with parallel system just unplug harness from GP relay.

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1983 Mercedes 300SD
1987 Mercedes 300SDL
2001 VW Passat 2.8 AWD
2007 OM642 Jeep WK 4x4