Two current flow diagrams from the > 05.2005 models and the rest are for later models.
You might want to consider replacing the roof mounted antenna with a new standard whip (you're not using the GPS on the roof with the RNS 510 are you??) or get an adapter for the RNS 510 and fit either a sharkfin with GPS or a whip and GPS antenna from a later model.
Of course first I would try joining them together but I'm not sure this would work. I wouldn't damage the connectors to test that out however.
You might want to consider replacing the roof mounted antenna with a new standard whip (you're not using the GPS on the roof with the RNS 510 are you??) or get an adapter for the RNS 510 and fit either a sharkfin with GPS or a whip and GPS antenna from a later model.
Of course first I would try joining them together but I'm not sure this would work. I wouldn't damage the connectors to test that out however.