I have not done an install, but I have re-purposed this style of bulkhead to another housing. They suck.
Vivid have a "Sea & Sea strobe bulkhead cap replacement". Hopefully you purchased this? Based upon marketing materials, this should simply screw into one of your 2 Nikonos style bulkhead connectors. But this will not work properly until you remove the internal guts of one of the bulkhead connectors to create a hole into the housing.
The only DIY needed is to remove the guts of the bulkhead connector. A brilliant lazy moron might try drilling through the guts? They are just plastic.
Your dx d200 has a screwy Sea & Sea dual surface mount bulkhead. It has a unique design feature in that it requires zero internal clearance.
If you are lucky, from the inside of the housing you will be able to access the circlip that holds the connector guts inplace. Cut the wires, pry the circlip off and push the connector guts out of the housing (from the inside). You are now ready to go and have not compromised any watertight seals. Consider buying several lottery tickets.
If you can not access the circlip, you will need to disassemble the dual bulkhead connector. At this point, you may want to be a moron and just drill it out? If not, then spin the ring off of each bulkhead connector - insert a very small screwdriver or drill bit into one of the 2 holes in the ring and push counter clockwise. Remove both rings. Then unscrew the 2 philips screws holding the face plate in place. Remove the face plate. Chop all of the wires off of the bulkhead you intend to sacrifice. From inside the housing, push that bulkhead outwards. You have now compromised a water tight seal. Remove the circlip and toss the innards. Res-assemble and hope things are still water tight.
Enjoy...