Looking at Brian's wiring diagram, it does not look like the tail lamp has anything to do with a spark, however, it does show a ground wire, common to all of the magneto internals, connected to ignition coil ground and frame ground. If this connection is bad, then grounding any of the other coil outputs would provide ground through that coil. This would result in a spark, but a weaker than normal spark.
Print out Brian's shematic and verify that the magneto ground is connected to frame ground and the coil as shown. Hopefully, the colors match, which should allow you to verrify your connections.