Chris:
There are a couple of possible scenarios to rig a MercMonitor into your setup. The fact that you have a junction box in place (with at least one open post) greatly simplifies the situation!
I'd recommend that you give me a call, and we can discuss the options. It's logistically possible to add a MercMonitor (though it becomes a little tricky with the actual configuration- I'll be glad to walk you through that).
Or... you can replace the System Tach & Speed units with a MercMonitor, and a System-Link Tach (all other instruments remain).
I'll be happy to discuss the Pros/Cons of both.
BTW... the connection harness is included with the MercMonitor.
