Mas Dubh :

Sorry not clear enough the the Navman 3100 repeater has the
following wires:-
Red: +ve 12v
Black: -ve and common nmea
Orange and Blue: navbus + /- not used
White: nmea input 1 - (think I used this one as a logical step)
Yellow: nmea input 2
Green: nmea input 3

Yuck! Sharing the ground on all inputs is a receipe for trouble. Nmea
devices are supposed to use differential signaling. I am not sure it is
necessary, but you may have to connect the B/- port of ISO OUT1 to the
ground, and, if you plug another device in parallel, make sure that it
either works in differential signaling (two input wires, separate from the
ground), or uses the same ground convention (and finally pray that you don't
fry the damned thing).

No mention in the bumpf of nmea (0183) speed!

Then it should be 4800, which is the speed defined in the standard.

I should also like to send waypoint and routes to the Garmin via NDC4
(that may be a hassle switching from nmea to Garmin protocols) from the
Tosh or maybe to the 3100 repeater if it's more legible and simpler.

You should check what kind of input your GPS expects. Ideally, it would just
be nmea at 4800 bps (with specialised sentences), in which case all you need
to do is connect the Garmin's input to ISO OUT 1. But it may be that it
expects RS232 at an exotic speed, in which case you would temporarily have
to connect it directly to your PC.